"'Hoots, mon!' is an all-purpose mild exclamation: 'Hoots' originally
functioning as a form of pejorative, like 'Nonsense!' or 'Rubbish!'

'Mon!' simply means man.

So originally, the exclamation would have served to point out to the
addressee that he (I've never heard of 'Hoots, womon!') was talking rubbish (or muck).

HOWEVER - the expression has become fully Lauderised, usually employed as a form of greeting: ('Hoots mon! Och Aye the Noo! It's a braw bright moonlicht nicht tae go roamin in the gloamin!') and as such has attained the status of such sub-DC Thomson utterances as 'Jings!' 'Crivvens!' or 'Help ma Boab!'

All of which are used to stereotype Scottish Vernacular and none of which
would ever be used by any self-respecting Scot this side of a Sunday Post. "