na-adjective
plausible but untrue; specious; seemingly truthful
Describes a statement, story, or claim that sounds believable but is actually false or deceptive. Often used with words like 嘘 (lie), 話 (story), or 言い訳 (excuse).
彼はまことしやかな嘘をついた。
He told a plausible lie.
その話はまことしやかに聞こえたが、全部
The story sounded credible, but it was all made up.
Rare kanji spelling; the kana form is strongly preferred.
もっともらしい also means 'plausible' or 'specious', but it often implies a more reasoned or logical appearance, while まことしやか emphasizes a deceptive truthfulness.
真実味 is a noun meaning 'air of truth' or 'verisimilitude', often used in phrases like 真実味がある (to have an air of truth). まことしやか is an adjectival noun describing the quality itself.
Derived from 真 (まこと, truth) + しやか, a suffix that adds a sense of 'seeming' or 'appearing'. The exact origin of しやか is uncertain, but it may be related to the classical auxiliary し (emphatic) and やか (adjectival suffix).