weak

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Lacking physical strength, energy, or vigor; feeble.
• adjective Likely to fail under pressure, stress, or strain; lacking resistance.
• adjective Lacking firmness of character or strength of will.
• adjective Lacking intensity or strength; faint.
• adjective Lacking the proper strength or amount of ingredients.
• adjective Having low prices or few transactions.
• adjective Lacking the ability to function normally or fully.
• adjective Unable to digest food easily; readily nauseated.
• adjective Lacking or resulting from a lack of intelligence.
• adjective Lacking aptitude or skill.
• adjective Lacking persuasiveness; unconvincing.
• adjective Lacking authority or the power to govern.
• adjective Of, relating to, or being those verbs in Germanic languages that form a past tense and past participle by means of a dental suffix, as start, started; have, had; bring, brought.
• adjective Of, relating to, or being the inflection of nouns or adjectives in Germanic languages with a declensional suffix that historically contained an n.
• adjective Unstressed or unaccented in pronunciation or poetic meter. Used of a word or syllable.
• adjective Designating a verse ending in which the metrical stress falls on a word or syllable that is unstressed in normal speech, such as a preposition.

from The Century Dictionary.

• To make weak; weaken.
• To soften.
• To become weak.
• Bending under pressure, weight, or force; pliant, or pliable; yielding; lacking stiffness or firmness: as, the weak stem of a plant.
• Lacking strength; not strong.
• Deficient in bodily strength, vigor, or robustness; feeble, either constitutionally or from age, disease, etc.; infirm; of the organs of the body, deficient in functional energy, activity, or the like: as, a weak stomach; weak eyes.
• Lacking moral strength or firmness; liable to waver or succumb when urged or tempted; deficient in steady principle or in force of character.
• Lacking mental power, ability, or balance; simple; silly; foolish.
• Unequal to a particular need or emergency; ineffectual or inefficacious; inadequate or unsatisfactory; incapable; impotent.
• Incapable of support; not to be sustained or maintained: unsupported by truth, reason, or justice: as, a weak claim, assertion, argument, etc.
• Deficient in force of utterance or sound; having little volume, loudness, or sonorousness; low; feeble; small.
• Not abundantly or sufficiently impregnated with the essential, required, or usual ingredients, or with stimulating or nourishing substances or properties; not of the usual strength: as, weak tea; weak broth; a weak infusion; weak punch.
• Deficient in pith, pregnancy, or point; lacking in vigor of expression: as, a weak sentence; a weak style.
• Resulting from or indicating lack of judgment, discernment, or firmness; arising from want of moral courage, of self-denial, or of determination; injudicious: as, a weak compliance; a weak surrender.
• Slight; inconsiderable; trifling.
• (I) In grammar, infiected— as a verb, by regular syllabic addition instead of by change of the radical vowel;
• as a noun or an adjective, with less full or original differences of case-and number-forms: opposed to strong (which see).
• Poorly supplied; deficient: as, a hand weak in trumps.
• Tending downward in price: as, a weak market; corn was weak.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Wanting physical strength.
• adjective Deficient in strength of body; feeble; infirm; sickly; debilitated; enfeebled; exhausted.
• adjective Not able to sustain a great weight, pressure, or strain.
• adjective Not firmly united or adhesive; easily broken or separated into pieces; not compact.
• adjective Not stiff; pliant; frail; soft.
• adjective Not able to resist external force or onset; easily subdued or overcome.
• adjective Lacking force of utterance or sound; not sonorous; low; small; feeble; faint.
• adjective Not thoroughly or abundantly impregnated with the usual or required ingredients, or with stimulating and nourishing substances; of less than the usual strength.
• adjective Lacking ability for an appropriate function or office.
• adjective Not possessing or manifesting intellectual, logical, moral, or political strength, vigor, etc.
• adjective Feeble of mind; wanting discernment; lacking vigor; spiritless.