Entry from OED Online
mullet, n.9 DRAFT ENTRY June 2001
slang (humorous and freq. derogatory).
[Origin uncertain. Perh. an extended use of MULLET-HEAD n. 2; but Grand Royal (1995) No. 2 proposes a number of other, largely humorous origins, including a relationship with MULLET n.1 and with the obsolete MULLET v.
Apparently coined, and certainly popularized, by U.S. hip-hop group the Beastie Boys (see quot. 1994), who also publish Grand Royal.]
A hairstyle, worn esp. by men, in which the hair is cut short at the front and sides, and left long at the back.
1994 Mullet Head (song)