You use underarm to describe actions, such as throwing a ball, in which you do not raise your arm above your shoulder. an underarm throw. overarm Underarm is also an adverb. Practise throwing a ball underarm. overarm
Underarm means in or for the areas under your arms, where they are joined to your body. underarm deodorants. Underarm is also a noun. Wash the feet, underarms and body surface using a soap. = armpit
with hand brought forward and up from below shoulder level; "an underhand pitch"; "an underhand stroke"