weak has 15 distinct synonyms spread across 5 of its 12 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is strong.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
wanting in physical strength
“a weak pillar”
(used of vowels or syllables) pronounced with little or no stress
“a syllable that ends in a short vowel is a light syllable”
deficient in magnitude; barely perceptible; lacking clarity or brightness or loudness etc
“a faint outline”
6 of the 12 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for weak.
Synonyms, antonyms and the broader and narrower relations for weak come from Princeton University’s WordNet. Full sources and licences.