An adverb used to emphasize circumstances or reasons: in any case, anyhow, at any rate; also conveys 'after all' or 'as you know'.
何せ忙しい時期だから、返事が遅れるかもしれない。
In any case, it's a busy period, so my reply might be delayed.
彼は経験が浅い。何せ新人だから仕方がない。
He's inexperienced. After all, he's a newcomer, so it's understandable.
何せ安かったから試しに買ってみた。
Anyway, it was cheap, so I bought it to try.