Green’s Dictionary of Slang

(orig. US black) a trenchcoat.

1971		N. Smith Gumshoe (1998) 17: I hang my trench in there, next to my three-buttoned suit.
1980		E. Folb Runnin’ Down Some Lines 123: They put on a leather piece — else a trench.
2001		N. Barlay Hooky Gear 33: Even jacked up on platforms her eyes is barely above the trench which is her black shapeless garms.