Green’s Dictionary of Slang

(UK Und.) a pickpocket who specializes in robbing members of a church congregation.

a.1790		H.T. Potter New Dict. Cant (1795) n.p.: groaner and sigher wretches hired by methodists and others to attend their meetings for the purposes of fraud.
1809		G. Andrewes Dict. Sl. and Cant n.p.: Groaners a sort of wretches who are employed by methodists and others, to attend their meetings, for the purpose of sighing, and looking demure, in order to give a colour to the sermon of the fellow who preaches; in the mean time pick the pockets of the good-disposed persons who may be in the same pew with them. Likewise attend charity-sermons, and rob the congregations of their watches on their coming out of church; exchange their bad hats for good ones; steal prayer-books; &c. &c. [Ibid.] Sighers fellows who attend the methodist meetings for the purpose of robbing the congregation.
1821		Flash Dict. [as cit. 1809].
1835		G. Kent Modern Flash Dict. [as cit. 1809].
1848		Flash Dict. in Sinks of London Laid Open.
1859		Matsell Vocabulum 39: groaners Thieves who attend at charity sermons, and rob the congregation of their watches and purses, exchange bad hats for good ones, steal the prayer-books, etc., etc.
1882		Dly Dispatch (Richmond, VA) 1 Nov. 3/3: ‘Groaners’ operate only in churches.