Green’s Dictionary of Slang

1. to have a nocturnal emission of semen, a ‘wet dream.’.

1942

(ref. to 1890s) W.L. McAtee Dial. Grant County Supplement 4: Dream off, v.phr., have an emission of semen while asleep [DARE].

2. to fall asleep on the job, to drift off.

1935

J. Conroy World to Win 201: Come on, sleepin’ Jesus! [...] Layin’ up there and dreamin’ off.