Green’s Dictionary of Slang

(UK society) a note attached to the name of a prospective male escort by a debutante or her mother.

2001		Indep. Rev. 21 May 16: They weren’t nearly as innocent as you might think back in the Thirties, what with their acronyms for untrustworthy men: NSIT (Not Safe in Taxis) or MTF (Must Touch Flesh).
2014		Guardian 5 May 🌐 My good friends [...] always made sure I didn't come to any harm with men [...] who had wandering-hand trouble. NSIT as we referred to them: Not Safe In Taxis.
2015		Dly Telegraph 24 Feb 🌐 Malcolm Muggeridge [...] was reportedly nicknamed ‘The Pouncer’ within the BBC and was also described as ‘a man fully deserving of the acronym NSIT – not safe in taxis’.