How Well Do You Know English Slang?
"British soldier," 1884, from Thomas Atkins, since 1815 the sample name for filling in army forms. Tommy gun (1929) is short for Thompson gun (see Thompson). Soon extended to other types of sub-machine gun, especially those favored by the mob.
tomboy: The red-haired tommy ... she'd put him over the jumps (1940s+)
A private in the British army: He met three Tommys in a bar (1884+)