expression, adverb
without reason; for no reason; without cause
Used adverbially to describe an action or state that occurs without any apparent reason or cause.
彼はわけもなく怒り出した。
He got angry for no reason.
わけもなく涙が出てきた。
Tears came to my eyes without any reason.
More literal and formal; わけもなく is more colloquial and often used for emotional or spontaneous actions.
From 訳 (わけ, 'reason') + も (inclusive particle) + なく (adverbial form of ない, 'not'). Literally 'without even a reason'.