Source : Webster's Revised Unabridged Dictionary (1913)

Guard \Guard\, n. [OF. guarde, F. garde; of German origin; cf.
   OHG. wart, marto, one who watches, mata a watching, Goth.
   wardja watchman. See {Guard}, v. t.]
   1. One who, or that which, guards from injury, danger,
      exposure, or attack; defense; protection.

            His greatness was no guard to bar heaven's shaft.
                                                  --Shak.

   2. A man, or body of men, stationed to protect or control a
      person or position; a watch; a sentinel.

            The guard which kept the door of the king's house.
                                                  --Kings xiv.
                                                  27.

   3. One who has charge of a mail coach or a railway train; a
      conductor. [Eng.]

   4. Any fixture or attachment designed to protect or secure
      against injury, soiling, or defacement, theft or loss; as:
      (a) That part of a sword hilt which protects the hand.
      (b) Ornamental lace or hem protecting the edge of a
          garment.
      (c) A chain or cord for fastening a watch to one's person
          or dress.
      (d) A fence or rail to prevent falling from the deck of a
          vessel.
      (e) An extension of the deck of a vessel beyond the hull;
          esp., in side-wheel steam vessels, the framework of
          strong timbers, which curves out on each side beyond
          the paddle wheel, and protects it and the shaft
          against collision.
      (f) A plate of metal, beneath the stock, or the lock
          frame, of a gun or pistol, having a loop, called a
          bow, to protect the trigger.
      (g) (Bookbinding) An interleaved strip at the back, as in
          a scrap book, to guard against its breaking when
          filled.

   5. A posture of defense in fencing, and in bayonet and saber
      exercise.

   6. An expression or admission intended to secure against
      objections or censure.

            They have expressed themselves with as few guards
            and restrictions as I.                --Atterbury.

   7. Watch; heed; care; attention; as, to keep guard.

   8. (Zo["o]l.) The fibrous sheath which covers the phragmacone
      of the Belemnites.

   Note: Guard is often used adjectively or in combination; as,
         guard boat or guardboat; guardroom or guard room; guard
         duty.

   {Advanced guard}, {Coast guard}, etc. See under {Advanced},
      {Coast}, etc.

   {Grand guard} (Mil.), one of the posts of the second line
      belonging to a system of advance posts of an army.
      --Mahan.

   {Guard boat}.
      (a) A boat appointed to row the rounds among ships of war
          in a harbor, to see that their officers keep a good
          lookout.
      (b) A boat used by harbor authorities to enforce the
          observance of quarantine regulations.

   {Guard cells} (Bot.), the bordering cells of stomates; they
      are crescent-shaped and contain chlorophyll.

   {Guard chamber}, a guardroom.

   {Guard detail} (Mil.), men from a company regiment etc.,
      detailed for guard duty.

   {Guard duty} (Mil.), the duty of watching patrolling, etc.,
      performed by a sentinel or sentinels.

   {Guard lock} (Engin.), a tide lock at the mouth of a dock or
      basin.

   {Guard of honor} (Mil.), a guard appointed to receive or to
      accompany eminent persons.

   {Guard rail} (Railroads), a rail placed on the inside of a
      main rail, on bridges, at switches, etc., as a safeguard
      against derailment.

   {Guard ship}, a war vessel appointed to superintend the
      marine affairs in a harbor, and also, in the English
      service, to receive seamen till they can be distributed
      among their respective ships.

   {Life guard} (Mil.), a body of select troops attending the
      person of a prince or high officer.

   {Off one's guard}, in a careless state; inattentive;
      unsuspicious of danger.

   {On guard}, serving in the capacity of a guard; doing duty as
      a guard or sentinel; watching.

   {On one's guard}, in a watchful state; alert; vigilant.

   {To mount guard} (Mil.), to go on duty as a guard or
      sentinel.

   {To run the guard}, to pass the watch or sentinel without
      leave.

   Syn: Defense; shield; protection; safeguard; convoy; escort;
        care; attention; watch; heed.

Source : WordNet®

off one's guard
     adj : not prepared or vigilant; "the blow caught him napping";
           "caught in an off-guard moment"; "found him off his
           guard" [syn: {napping}, {off-guard(a)}, {off guard(p)},
            {off one's guard(p)}, {off his guard}, {off her guard},
            {off your guard}]