To untruss a point; to let down one’s breeches in order to ease one’s self. Breeches were formerly tied with points, which till lately were distributed to the boys every Whit Monday by the churchwardens of most of the parishes in London, under the denomination of tags: these tags were worsteds of different colours twisted up to a size somewhat thicker than packthread, and tagged at both ends with tin. Laces were at the same given to the girls.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Unrigged * UntwistedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes