Green’s Dictionary of Slang

1. a servant, esp. an under-servant who assists another.

1937

Partridge DSUE (1984) 1194: —1857.

2. one who ‘tags along’ behind another person, esp. for detective or spying purposes.

1888		N.Y. Worrld c.3 Dec. q. in Evans & Skinner Jack the Ripper Sourcebook (2000) 620: Inspector Byrnes was asked what his object was in shadowing Twomblety. ‘I simply wanted to put a tag on him,’ he replied, ‘so that we can tell where he is’.
1966		‘A. Hall’ 9th Directive 62: Who were the tags? [...] The thin one, and the one with the splay-footed walk — and noone else .