干 drew a forked pole — weapon and shield of the earliest battles (干戈, 'shields and dagger-axes', still means warfare). Simplification later merged two other characters into it: 乾 (dry) and 幹 (to do), giving three ancient words one three-stroke body.
干 works two shifts: gàn is the muscular 'do' — 干活 (do manual work), 干什么? ('whatcha doing?' — or, sharper, 'what do you think you're doing?!'), 能干 (capable). gān is dry — 干杯 ('dry the cup': cheers!), 饼干 (biscuits: 'dry cakes'). Tone is everything: 干杯 toasts a friendship, 干什么 can start an argument. Three strokes, from battlefield to bar.
The forked tool in hand — DO the work (gàn), till the field is DRY (gān).
干戈 'shield and dagger-axe' still means 'war' — 化干戈为玉帛, 'turn weapons into jade and silk' = make peace.