adjective

1. 1. lacking in strength of body or muscle; not physically strong
   2. lacking vitality; feeble; infirm
2. lacking in skill or strength in combat or competition a weak team
3. lacking in moral strength or willpower; yielding easily to temptation, the influence of others, etc.
4. lacking in mental power, or in the ability to think, judge, decide, etc.
5. 1. lacking ruling power, or authority a weak government
   2. having few resources; relatively low in wealth, numbers, supplies, etc. the weaker nations
6. lacking in force or effectiveness weak discipline
7. 1. lacking in strength of material or construction; unable to resist strain, pressure, etc.; easily torn, broken, bent, etc. a weak railing
   2. not sound or secure; unable to stand up to an attack a weak fortification
8. 1. not functioning normally or well: said of a body organ or part weak eyes
   2. easily upset; queasy a weak stomach
9. indicating or suggesting moral or physical lack of strength weak features
10. lacking in volume, intensity, etc.; faint a weak voice, a weak current
11. lacking the usual or proper strength; specif.,
    1. having only a small amount of its essential ingredient; diluted weak tea
    2. not as potent as usual or as others of the kind a weak drug
    3. lacking, poor, or deficient in something specified weak in grammar, a baseball team weak in pitchers
12. 1. ineffective; unconvincing a weak argument
    2. faulty weak logic
13. tending toward lower prices: said of a market, stock, etc.
14. Chem. having a low ion concentration: said as of certain acids and bases
15. Gram. expressing variation in tense by the addition of an inflectional suffix rather than by internal change of a syllabic vowel; regular (Ex.: talk, talked, talked)
16. Phonet. unstressed or lightly stressed: said of a syllable
17. Photog. thin (sense )
18. Prosody designating or of a verse ending in which the stress falls on a word or syllable that is normally unstressed

Etymology: ME waik < ON veikr, akin to OE wac, feeble (which the ON word replaced) < IE *weig-, *weik- (< base *wei-, to bend) > week, wicker, L vicis, change