First recorded in 1893 in the UK. The idea is that a busman, to go off on a holiday, would take an excursion by bus, thereby engaging in a similar activity to his work.
busman's holiday (plural busman's holidays)
- (idiomatic) A holiday or vacation during which one does something similar to what one does as work.
- "Busman's holiday" in Michael Quinion, Ballyhoo, Buckaroo, and Spuds, 2004.