Green’s Dictionary of Slang

(UK Und.) a method of thieving whereby someone enters a house posing as a servant later to decamp with whatever they can steal.

1718		C. Hitchin Regulator 19: The Service-Lay, alias to hire one-self for a Servant, and to rob the House.
1768		(con. 1710–25) Tyburn Chronicle II in Groom (1999) xxvii: The Service Lay Hiring themselves for Servants in order to rob the House.
1786		Whole Art of Thieving .