Green’s Dictionary of Slang

mill n.2

[abbr.]
(US)

1. [mid-19C; 1940s+] a millimetre, esp. in the diameter of a tube or gun barrel [Latin, mille, thousand].

2. [1910s+] a million, usu. dollars.

3. [1960s] (drugs) $1000 worth of heroin [Lat., mille, thousand].