đầu tắt mặt tối 
to work very hard [ đầu ] : (1) beginning, first; (2) head, mind, pole, end, hood (of a car)
[ tắt ] : (1) to be shortened, abbreviated; (2) to turn off, extinguish, switch off, turn off; (3) to die out, be extinct
[ mặt ] : (1) right; (2) face, surface, dial; (3) side, aspect
[ tối ] : (1) night, evening, dark; (2) very
Most Common Compounds Set 1:
Most Common Compounds Set 2 :