head down, bum up

listen to the pronunciation of head down, bum up
Englisch - Englisch
A figurative bodily position adopted when working hard at some task, either physical or mental. (Head down may well be literal, but bum up is figurative.)

2007: Hadn't heard a thing. Hard at work, head down, bum up. — Gold Coast Bulletin, Call of sea sirens, 6 April 2007.