Green’s Dictionary of Slang

ass v.

[ass n. (3)]

to work as a prostitute.

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[US]W.T. Vollmann Whores for Gloria 139: Assing – Tricking.

In phrases

ass up (v.)

(US) to prepare, get ready; spec. to mount of motorcycle.

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[US]J. Dobyns No Angel n.p.: As we assed up, I looked at my guys [...] There was no way a roomful of Angels was going to outdo [...] us [...] We went outside, assed up, and started our engines.