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.
head down
الواصلة
head Down
التركية النطق
hed daun
النطق
/ˈhed ˈdoun/ /ˈhɛd ˈdaʊn/
علم أصول الكلمات
[ 'hed ] (noun.) before 12th century. Middle English hed, from Old English hEafod; akin to Old High German houbit head, Latin caput.