the.com/underarm

means Having to do with the armpit — the region or motion beneath the shoulder joint, as in underarm deodorant or an underarm throw.

from A plain compound from under + arm, tracking exactly what it describes — the space or surface beneath the arm. English loves this kind of honest carpentry-word, stacking two simple pieces to name a spot the body already knew about long before anyone needed to sell deodorant for it. The throwing sense (an 'underarm pitch') comes later, describing a motion that keeps the arm below shoulder height rather than the armpit itself.