Perhaps the term 'hairy' stems from the days in the late '60s when hossifers wore smooth barathea battledress (incuding the 'aircrew version' for Lightning and V-force mates - best uniform ever!) whilst NCOs and ORs wore 'hairy' wool serge battledress? Someone who'd spent a few years itching in the woolly stuff was thus an 'ex-hairy'?
That 'aircrew' No 2 battledress was infinitely smarter than the awful 'Thunderbirds' smock which came into use around 1974 - but both were better than the tatty prep-school pullover now in use!