Source : Webster's Revised Unabridged Dictionary (1913)

Bug \Bug\, n. [OE. bugge, fr. W. bwg, bwgan, hobgoblin,
   scarecrow, bugbear. Cf. {Bogey}, {Boggle}.]
   1. A bugbear; anything which terrifies. [Obs.]

            Sir, spare your threats: The bug which you would
            fright me with I seek.                --Shak.

   2. (Zo["o]l.) A general name applied to various insects
      belonging to the Hemiptera; as, the squash bug; the chinch
      bug, etc.

   3. (Zo["o]l.) An insect of the genus {Cimex}, especially the
      bedbug ({C. lectularius}). See {Bedbug}.

   4. (Zo["o]l.) One of various species of Coleoptera; as, the
      ladybug; potato bug, etc.; loosely, any beetle.

   5. (Zo["o]l.) One of certain kinds of Crustacea; as, the sow
      bug; pill bug; bait bug; salve bug, etc.

   Note: According to present popular usage in England, and
         among housekeepers in America, bug, when not joined
         with some qualifying word, is used specifically for
         bedbug. As a general term it is used very loosely in
         America, and was formerly used still more loosely in
         England. ``God's rare workmanship in the ant, the
         poorest bug that creeps.'' --Rogers (--Naaman). ``This
         bug with gilded wings.'' --Pope.

   {Bait bug}. See under {Bait}.

   {Bug word}, swaggering or threatening language. [Obs.]
      --Beau. & Fl.

Source : WordNet®

bug
     v 1: annoy persistently; "The children teased the boy because of
          his stammer" [syn: {tease}, {badger}, {pester}, {beleaguer}]
     2: tap a telephone or telegraph wire to get information; "The
        FBI was tapping the phone line of the suspected spy"; "Is
        this hotel room bugged?" [syn: {wiretap}, {tap}, {intercept}]
     [also: {bugging}, {bugged}]

bug
     n 1: general term for any insect or similar creeping or crawling
          invertebrate
     2: a fault or defect in a system or machine [syn: {glitch}]
     3: a small hidden microphone; for listening secretly
     4: insects with sucking mouthparts and forewings thickened and
        leathery at the base; usually show incomplete
        metamorphosis [syn: {hemipterous insect}, {hemipteran}, {hemipteron}]
     5: a minute life form (especially a disease-causing bacterium);
        the term is not in technical use [syn: {microbe}, {germ}]
     [also: {bugging}, {bugged}]

Source : Free On-Line Dictionary of Computing

bug
     
         An unwanted and unintended property of a program
        or piece of hardware, especially one that causes it to
        malfunction.  Antonym of {feature}.  E.g. "There's a bug in
        the editor: it writes things out backward."  The
        identification and removal of bugs in a program is called
        "{debugging}".
     
        Admiral {Grace Hopper} (an early computing pioneer better
        known for inventing {COBOL}) liked to tell a story in which a
        technician solved a {glitch} in the {Harvard Mark II machine}
        by pulling an actual insect out from between the contacts of
        one of its relays, and she subsequently promulgated {bug} in
        its hackish sense as a joke about the incident (though, as she
        was careful to admit, she was not there when it happened).
        For many years the logbook associated with the incident and
        the actual bug in question (a moth) sat in a display case at
        the Naval Surface Warfare Center (NSWC).  The entire story,
        with a picture of the logbook and the moth taped into it, is
        recorded in the "Annals of the History of Computing", Vol. 3,
        No. 3 (July 1981), pp. 285--286.
     
        The text of the log entry (from September 9, 1947), reads
        "1545 Relay #70 Panel F (moth) in relay.  First actual case of
        bug being found".  This wording establishes that the term was
        already in use at the time in its current specific sense - and
        Hopper herself reports that the term "bug" was regularly
        applied to problems in radar electronics during WWII.
     
        Indeed, the use of "bug" to mean an industrial defect was
        already established in Thomas Edison's time, and a more
        specific and rather modern use can be found in an electrical
        handbook from 1896 ("Hawkin's New Catechism of Electricity",
        Theo. Audel & Co.)  which says: "The term "bug" is used to a
        limited extent to designate any fault or trouble in the
        connections or working of electric apparatus."  It further
        notes that the term is "said to have originated in
        {quadruplex} telegraphy and have been transferred to all
        electric apparatus."
     
        The latter observation may explain a common folk etymology of
        the term; that it came from telephone company usage, in which
        "bugs in a telephone cable" were blamed for noisy lines.
        Though this derivation seems to be mistaken, it may well be a
        distorted memory of a joke first current among *telegraph*
        operators more than a century ago!
     
        Actually, use of "bug" in the general sense of a disruptive
        event goes back to Shakespeare!  In the first edition of
        Samuel Johnson's dictionary one meaning of "bug" is "A
        frightful object; a walking spectre"; this is traced to
        "bugbear", a Welsh term for a variety of mythological monster
        which (to complete the circle) has recently been reintroduced
        into the popular lexicon through fantasy {role-playing games}.
     
        In any case, in jargon the word almost never refers to
        insects.  Here is a plausible conversation that never actually
        happened:
     
        "There is a bug in this ant farm!"
     
        "What do you mean?  I don't see any ants in it."
     
        "That's the bug."
     
        [There has been a widespread myth that the original bug was
        moved to the Smithsonian, and an earlier version of this entry
        so asserted.  A correspondent who thought to check discovered
        that the bug was not there.  While investigating this in late
        1990, your editor discovered that the NSWC still had the bug,
        but had unsuccessfully tried to get the Smithsonian to accept
        it - and that the present curator of their History of
        American Technology Museum didn't know this and agreed that it
        would make a worthwhile exhibit.  It was moved to the
        Smithsonian in mid-1991, but due to space and money
        constraints has not yet been exhibited.  Thus, the process of
        investigating the original-computer-bug bug fixed it in an
        entirely unexpected way, by making the myth true!  - ESR]
     
        [{Jargon File}]
     
        (1999-06-29)