to use one's head

to slice off, to chip off / (figuratively) to weaken, to dampen, to thin down

kitchen knife, cooking knife, cleaver

to use / to employ / to make use of, to apply / to spend

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

魚: fish

(idiomatic) the fish rots from the head

use one thing for many purposes / work someone hard