a bath; taking a bath
Often used with verbs like 浴びる or 入る, implying a quick or single bath. The prefix 一 adds a sense of 'one' or 'a single'.
仕事の後で一風呂浴びたい。
I want to take a bath after work.
彼は一風呂浴びてから夕食を食べた。
He took a bath and then had dinner.
Compound of 一 (one) and 風呂 (bath). The exact historical derivation is straightforward.