Named after the technique used to make Mr Ed (the famous 'talking horse' talk. Originally there were two mouth movements that Mr Ed could make. There was the "Warm Wilbur", which made Mr Ed's lips move a little (as if he's saying 'Wilbur'), and the "Cold Wilbur", where his lips move a lot and he's saying 'Nay!'. The Warm Wilbur was accomplished by inserting a warm finger into the horses anus, and the Cold Wilbur was inserting a finger that had been kept in an ice-cold-bowl-of-water into the horses anus (incurring a more extreme reaction).
In modern parlance, a Cold Wilbur is the act of inserting an extremely cold finger into (normally) another person's anus.
Give him the cold finger? I gave him a Cold Wilbur!