also: せよ
even if; even though; granted that
Concessive expression meaning 'even if it is the case that...' or 'granted that...'. Used to acknowledge a condition while still asserting the main point. Often follows nouns, na-adjectives, or the plain form of verbs/adjectives.
たとえ子供にせよ、そんなことをしてはいけない。
Even if you are a child, you must not do such a thing.
理由が何にせよ、遅刻は遅刻だ。
Whatever the reason may be, being late is being late.
にせよ is a stiff, literary variant of にしても, common in formal writing rather than casual speech.
Derived from the classical copula なり (に) combined with the imperative form of する (せよ), literally 'let it be that...'. The exact historical development is uncertain, but it functions as a concessive particle in modern Japanese.