Green’s Dictionary of Slang

tap (on the shoulder) n.

an arrest.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		‘Jon Bee’ A Dict. of the Turf, The Ring, The Chase, etc.
1951		Montana Standard (Butte, MO) 3 Sept. 6/1: Harry doesn’t count the years he spent waiting for that tap on the shoulder from the time he went over the wall.
1978		Vancouver Sun 28 Mar. 19/3-4: The editor of the Pretoria News (Sth Africa) told me: Someday the tap on the shoulder will come for you or your son or daughter. We [i.e. South Africa] are a society ruled by a tap on the shoulder.