A small raised thing made to be pressed or fastened. Etymologically a bud - something that pushes out from a surface.
Old French boton (a bud, a knob), from boter (to thrust out, to bud), of Frankish origin; a button is, at the root, a small bud pushed out from a surface to be pressed.
A small raised thing made to be pressed — the same image, a bud breaking a surface, sits behind both the sewn fastener and the pressed control.