Someone who hides under an office desk with out informing the desk's owner that they are there. Being a desk rabbit can have many benefits, one can listen into office gossip, find out who the boss is going to fire next or even look up the secretary's skirt.
Moss: Roy's stuck underneath a lady's desk.
Jen: What, still?
Moss: Look, I know that normally this would be very funny but he's been under there too long for a reasonable explanation. If that woman looks down shes going to assume he's a desk rabbit.