"one who commits violent robbery," 1865, agent noun from mug (v.1).
Entries linking to mugger
mug v.1
"to beat up," 1818, originally "to strike the face" (in pugilism), from mug (n.2) "face." The general meaning "attack" is attested by 1846, and "attack to rob" by 1864. Perhaps influenced by thieves' slang mug "dupe, fool, sucker" (1851). Related: Mugged; mugging.