Green’s Dictionary of Slang

1. (Aus.) a girl- or boyfriend; also as adj.

1956		J.E. Webs So much for Sydney 64: Our Girls are most ‘teenagers’, and it doesn’t matter whether they are socialites, shopgirls or ‘tagalong tarts’ [AND].
1963		Bulletin (Sydney) 23 Nov. 16/3: Tag-a-long, a girlfriend (or boyfriend).

2. an associate, a parasite.

1986		J. Ellroy Suicide Hill 51: ‘I know two Chicano dudes, brothers, who're looking for work. Smart—one righteous vato, one tagalong’.
2012		R.A. Dickey Wherever I Wind Up 58: While most of my friends meet with their parents and go out for dinner or dessert, I am a tagalong.
2020		G. Armstrong Young Team 45: They’ve git another wee entourage roon them ae wee tag-alongs n tramps fae other schemes.