without reason; for no reason; without cause
Used adverbially to describe an action or state that occurs without any apparent reason or cause.
He got angry for no reason.
Tears came to my eyes without any reason.
Kanji form using 無; rare and somewhat literary.
From 訳 (わけ, 'reason') + も (inclusive particle) + なく (adverbial form of ない, 'not'). Literally 'without even a reason'.