Green’s Dictionary of Slang

dancer n.

1. a cat burglar who ‘dances’ along the roof and in through a convenient window, or a thief entering a house to rob the upstairs, when the residents are not in bed, or are out; thus usu. a daylight robbery; 20C use also refers to those who steal from empty offices [SE dance, later strengthened by dance v. (3)].

[UK]B.M. Carew ‘The Oath of the Canting Crew’ in Farmer Musa Pedestris (1896) 51: No dimber, dambler, angler, dancer, / Prig of cackler, prig of prancer.
[UK]Hotten Sl. Dict.
[Aus]Australiasian (Melbourne) 17 July 8/5: A man who gets over roofs or in at windows is called a garreter and a dancer.
[UK]Sl. Dict.
[UK]Clarkson & Richardson Police! 260: About 40 were burglars, ‘dancers,’ ‘garreters,’ and others adept with the skeleton keys.
[US](con. 1910s) D. Mackenzie Hell’s Kitchen 123: Expert ‘dancers’ are fast workers. They have to be, since they are doing their job in the dangerous hours of daylight.

2. one who copulates [dance v. (1)].

[UK]Crim.-Con. Gaz. 26 Jan. 33/2: One afternoon when Lady Sykes was taking her afternoon’s nap, Sir freancis was entertaining the maid. When it was all over, the girl, quite proud, said to him, ‘Sir, which is the best dancer, my mistress or me?’.