Green’s Dictionary of Slang

(US) an excuse.

1825		N.-Y. National Advocate 28 May 2/3: On coming before the Judge again, she exulted at having ‘outwitted him,’ as she termed it, and asked him what ‘get off’ he would have now.
1842		Whip & Satirist of NY & Brooklyn (NY) 16 Apr. n.p.: Plead it as a get-ff from a scared obligation.