Green’s Dictionary of Slang

(US Und./police/prison) to act as a courier of contraband, esp. illicit drugs; thus muling n.

1965		in Sat. Eve. Post 4 Dec. 26: ‘Muling’ a term which stems from the days when Mexican peons smuggled marijuana, hidden under heads of lettuce and other produce, on the backs of mules [HDAS].
1990		S. Morgan Homeboy 339: ‘What do you need me for?’ ‘ To mule em [i.e. contraband items] through the gates.’.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Muling: Using devices to move contraband from one location to another.
2006		G. Pelecanos Night Gardener 132: He mulin some dope back from New York tonight.