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Synonyms for Weak (same or very similar meaning)

WordNet sense 1 (being or moving lower in position or less in some value):
down

WordNet sense 2 (capable of being wounded or hurt):
vulnerable

WordNet sense 3 (inferior in strength or significance):
puny

WordNet sense 4 ((of light) lacking in intensity or brightness; dim or feeble):
pale

WordNet sense 5 (weak in willpower, courage or vitality):
spineless

WordNet sense 6 (lacking solidity or strength and liable to break):
flimsy, fragile

WordNet sense 7 (having little substance or significance):
slight, thin

WordNet sense 8 (lacking in strength or firmness or resilience):
flaccid, limp

WordNet sense 9 (pathetically lacking in force or effectiveness):
feeble, lame, lame

WordNet sense 10 (lacking strength or vigor):
faint

WordNet sense 11 (lacking strength or vigor):
enervated, debilitated, asthenic

WordNet sense 12 (used of vowels or syllables; pronounced with little or no stress):
light, unaccented

WordNet sense 13 (not bearing a stress or accent):
unstressed

WordNet sense 14 (not having or showing or requiring special skill or proficiency):
unskilled

WordNet sense 15 (lacking physical strength or vitality):
decrepit, infirm, rickety, sapless, weakly, debile

WordNet sense 16 (physically weak):
frail

WordNet sense 17 (lacking power):
powerless

WordNet sense 18 (capable of being perceived by the mind or senses):
perceptible

WordNet sense 19 (having the attributes of man as opposed to e.g. divine beings):
imperfect

WordNet sense 20 (having human form or attributes as opposed to those of animals or divine beings):
human

WordNet sense 21 (likely to fail or make errors):
fallible

WordNet sense 22 (overly diluted; thin and insipid):
washy, watery

WordNet sense 23 (reduced in strength or concentration or quality or purity):
dilute

WordNet sense 24 (lacking or marked by lack of intellectual acuity):
stupid


From the ODE community, based on WordNetadd/edit