Green’s Dictionary of Slang

drub off v.

(US) to masturbate oneself or another.

[US]P.C. Van Buskirk Erotic Diaries 76: Van Buskirk reported that he [sc. William McFarland] once offered to give Midshipman Lester Beardslee a good ‘drubbing off’, adding that Beardslee wisely refused [Simes:DLSS].