Belgian biscuit 

British. To quote the Profanisaurus, 'a vile and non-specific sex act performed by the staff of a rub-a-tug shop'.

Whatever it is it's rarely heard outside of Viz and, depending on exactly what you take it to mean, rarely seen either, thank Christ.

I offered him a Cleveland steamer and the filthy cunt's only asked if I do Belgian biscuit.

Belgian biscuit by Lord Grimcock September 11, 2007