Green’s Dictionary of Slang

G-string n.

[SE G-string, the minimal ‘loin-cloth’ worn by striptease artists etc]

1. (US black) any device – a tampon, towel etc. – used to staunch the flow of menstrual blood.

[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].

2. (US gay) an athletic supporter, a jockstrap.

[US]B. Rodgers Queens’ Vernacular 119: an athletic supporter [...] G-string.

In exclamations

don’t bust your G-string!

(Aus.) calm down!

J. Wynnum Tar Dust 32: ‘The filthy beast has no decency in him at all’ ‘Don’t bust your G string, Bert,’ soothed Toggle.