na-adjective, noun
Describes claims, stories, or ideas that lack any basis in fact or reason. Often used in formal or critical contexts. Frequently appears in the compound 荒唐無稽 (こうとうむけい, 'preposterous, absurd').
See also: 荒唐無稽
その噂は全くの無稽だ。
That rumor is completely unfounded.
彼の主張は無稽で、誰も信じなかった。
His claim was groundless, and no one believed it.
From Chinese 無稽 (wújī), meaning 'without basis' or 'unfounded'. The kanji 無 means 'without' and 稽 means 'to examine' or 'to check', so the literal sense is 'unexamined' or 'unverifiable'.