What is "untrussing"

Usage examples of "untrussing".

And Kang Xi, who, for all the Imperial gravity of his everyday demeanour, was still at heart a boy, gleefully took up the suggestion, hitched up his gown, and began untrussing his trousers.

Fumbling with cold hands in my breeches, I realised that the man next to me was untrussing likewise.