Green’s Dictionary of Slang

undub v.

(UK Und.) to unlock.

1753		J. Poulter Discoveries (1774) 43: Undub the Jeger, and jump the Glaze; open the Door, and lift up the Window.
1786		Whole Art of Thieving .
1807		H. Tufts Autobiog. (1930) 293: Undub the jigger signifies unlock the door.
1812		Vaux Vocab. of the Flash Lang.
1821		D. Haggart Autobiog. 28: I got a seat behind them, and succeeded in undubbing the stretch which slung the scout round her waist.
1833		H. Smith Gale Middleton 1 161: That’s Bob — I’ll undub the jigger.

In phrases

undub the qua (v.)

(US Und.) to escape from prison.

1807

H. Tufts Autobiog. (1930) 293: Undub the jigger . . . unlock the door. [...] Undub the qua . . . unlock the jail.