twitching arse hole

Or as my Grandad used to say..in moments of anguish. That made my 'sphincter twitch'

What do you mean Grandad?

Come on boy....you ever been so frightened that your arse hasn't done the 'half-crown, sixpence'

(This post is for Iain Dale when he googles 'half-crown, sixpence' that will be a real 3 shillings moment)