Wiktionary
let oneself go
vb. 1 (context idiomatic English) to cease to care about one's appearance 2 (context idiomatic English) to relax and enjoy oneself without restraint.
Usage examples of "let oneself go".
One can let oneself go and spread--you know, like taking a girdle off.
But one must be a flaming harlot -- one must let oneself go, whether one's star be twin with that of Shelly, or of Blake, or of Titian, or of Beethoven.
He was even starting to like freefallit could be quite relaxing if one let oneself go with it.