Antonyms of adj grammatical

1 of 2 senses of grammatical

Sense 2
grammatical (vs. ungrammatical), well-formed -- (conforming to the rules of grammar or usage accepted by native speakers; "spoke in grammatical sentences")

ungrammatical (vs. grammatical), ill-formed -- (not grammatical; not conforming to the rules of grammar or accepted usage)
        => incorrect -- ((of a word or expression) not agreeing with grammatical principles)

Similarity of adj grammatical

2 senses of grammatical

Sense 1
grammatical, grammatic -- (of or pertaining to grammar; "the grammatic structure of a sentence"; "grammatical rules"; "grammatical gender")

Sense 2
grammatical (vs. ungrammatical), well-formed -- (conforming to the rules of grammar or usage accepted by native speakers; "spoke in grammatical sentences")

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