Source : Webster's Revised Unabridged Dictionary (1913)

Weak \Weak\ (w[=e]k), a. [Compar. {Weaker} (-[~e]r); superl.
   {Weakest}.] [OE. weik, Icel. veikr; akin to Sw. vek, Dan. veg
   soft, flexible, pliant, AS. w[=a]c weak, soft, pliant, D.
   week, G. weich, OHG. weih; all from the verb seen in Icel.
   v[=i]kja to turn, veer, recede, AS. w[=i]can to yield, give
   way, G. weichen, OHG. w[=i]hhan, akin to Skr. vij, and
   probably to E. week, L. vicis a change, turn, Gr. e'i`kein to
   yield, give way. [root]132. Cf. {Week}, {Wink}, v. i.
   {Vicissitude}.]
   1. Wanting physical strength. Specifically:
      (a) Deficient in strength of body; feeble; infirm; sickly;
          debilitated; enfeebled; exhausted.

                A poor, infirm, weak, and despised old man.
                                                  --Shak.

                Weak with hunger, mad with love.  --Dryden.
      (b) Not able to sustain a great weight, pressure, or
          strain; as, a weak timber; a weak rope.
      (c) Not firmly united or adhesive; easily broken or
          separated into pieces; not compact; as, a weak ship.
      (d) Not stiff; pliant; frail; soft; as, the weak stalk of
          a plant.
      (e) Not able to resist external force or onset; easily
          subdued or overcome; as, a weak barrier; as, a weak
          fortress.
      (f) Lacking force of utterance or sound; not sonorous;
          low; small; feeble; faint.

                A voice not soft, weak, piping, and womanish.
                                                  --Ascham.
      (g) Not thoroughly or abundantly impregnated with the
          usual or required ingredients, or with stimulating and
          nourishing substances; of less than the usual
          strength; as, weak tea, broth, or liquor; a weak
          decoction or solution; a weak dose of medicine.
      (h) Lacking ability for an appropriate function or office;
          as, weak eyes; a weak stomach; a weak magistrate; a
          weak regiment, or army.

   2. Not possessing or manifesting intellectual, logical,
      moral, or political strength, vigor, etc. Specifically: 
      (a) Feeble of mind; wanting discernment; lacking vigor;
          spiritless; as, a weak king or magistrate.

                To think every thing disputable is a proof of a
                weak mind and captious temper.    --Beattie.

                Origen was never weak enough to imagine that
                there were two Gods.              --Waterland.
      (b) Resulting from, or indicating, lack of judgment,
          discernment, or firmness; unwise; hence, foolish.

                If evil thence ensue, She first his weak
                indulgence will accuse.           --Milton.
      (c) Not having full confidence or conviction; not decided
          or confirmed; vacillating; wavering.

                Him that is weak in the faith receive ye, but
                not to doubtful disputations.     --Rom. xiv. 1.
      (d) Not able to withstand temptation, urgency, persuasion,
          etc.; easily impressed, moved, or overcome;
          accessible; vulnerable; as, weak resolutions; weak
          virtue.

                Guard thy heart On this weak side, where most
                our nature fails.                 --Addison.
      (e) Wanting in power to influence or bind; as, weak ties;
          a weak sense of honor of duty.
      (f) Not having power to convince; not supported by force
          of reason or truth; unsustained; as, a weak argument
          or case. ``Convinced of his weak arguing.'' --Milton.

                A case so weak . . . hath much persisted in.
                                                  --Hooker.
      (g) Wanting in point or vigor of expression; as, a weak
          sentence; a weak style.
      (h) Not prevalent or effective, or not felt to be
          prevalent; not potent; feeble. ``Weak prayers.''
          --Shak.
      (i) Lacking in elements of political strength; not
          wielding or having authority or energy; deficient in
          the resources that are essential to a ruler or nation;
          as, a weak monarch; a weak government or state.

                I must make fair weather yet awhile, Till Henry
                be more weak, and I more strong.  --Shak.
      (k) (Stock Exchange) Tending towards lower prices; as, a
          weak market.

   3. (Gram.)
      (a) Pertaining to, or designating, a verb which forms its
          preterit (imperfect) and past participle by adding to
          the present the suffix -ed, -d, or the variant form
          -t; as in the verbs abash, abashed; abate, abated;
          deny, denied; feel, felt. See {Strong}, 19
      (a) .
      (b) Pertaining to, or designating, a noun in Anglo-Saxon,
          etc., the stem of which ends in -n. See {Strong}, 19
      (b) .

   Note: Weak is often used in the formation of self-explaining
         compounds; as, weak-eyed, weak-handed, weak-hearted,
         weak-minded, weak-spirited, and the like.

Weak \Weak\, v. t. & i. [Cf. AS. w?can. w[=a]cian. See {Weak},
   a.]
   To make or become weak; to weaken. [R.]

         Never to seek weaking variety.           --Marston.

Weak \Weak\, a.
   1. (Stock Exchange) Tending toward a lower price or lower
      prices; as, wheat is weak; a weak market.

   2. (Card Playing) Lacking in good cards; deficient as to
      number or strength; as, a hand weak in trumps.

   3. (Photog.) Lacking contrast; as, a weak negative.

Source : WordNet®

weak
     adj 1: having little physical or spiritual strength; "a weak radio
            signal"; "a weak link" [ant: {strong}]
     2: overly diluted; thin and insipid; "washy coffee"; "watery
        milk"; "weak tea" [syn: {watery}, {washy}]
     3: lacking power [syn: {powerless}] [ant: {powerful}]
     4: used of vowels or syllables; pronounced with little or no
        stress; "a syllable that ends in a short vowel is a light
        syllable"; "a weak stress on the second syllable" [syn: {unaccented},
         {light}]
     5: having the attributes of man as opposed to e.g. divine
        beings; "I'm only human"; "frail humanity" [syn: {fallible},
         {frail}, {imperfect}]
     6: lacking force; feeble; "a forceless argument" [syn: {forceless},
         {unforceful}] [ant: {forceful}]
     7: lacking physical strength or vitality; "a feeble old woman";
        "her body looked sapless" [syn: {decrepit}, {debile}, {feeble},
         {infirm}, {sapless}, {weakly}]
     8: used of verbs having standard (or regular) inflection
     9: lacking physical strength or vigor
     10: characterized by excessive softness or self-indulgence; "an
         effeminate civilization" [syn: {effeminate}]