Expression used by bar staff to describe a a beer tap for a product that is no longer in stock and has been so for an extended period. Generally the product in question will never be stocked again so it is only due to the idiotcy of management that the tap remains in place.
Customer: I'll have a pint of Becks.
Exasperated barman: Sorry but we've no Becks on draught.
Customer: But there's a tap there.
Increasingly irate barman: That tap is like a Eunuch's cock. I can pull at it all day and nothing's going to comeout.