Green’s Dictionary of Slang

(UK Und.) to be let into a house by a servant, telling him not to wake his sleeping master as you will wait, and proced to steal something.

1718		C. Hitchin Regulator 19: The Question-Lay, alias that is to knock at the Door, and ask for the Gentleman of the House, if a Bed you desire the Servant not to disturb him, but you will wait untill he rises, and then an opportunity to steal something.
1768		(con. 1710–25) Tyburn Chronicle II in Groom (1999) xxvii: The Question Lay To knock at a Door early in the Morning and ask for the Master of the House, and if he’s a Bed, to desire the Servant not to disturb him, for you’ll wait till he rises, and so you take an Opportunity of stealing something.
1786		Whole Art of Thieving .