A person of the criminal sort adept at finding his or her way into or out of a building. Usually someone of small stature able to fit through tight places.
Exemplified in Michael Crichton's "The Great Train Robbery" in the character of Clean Willy.
Pierce: "You say your the fastest, bloody best Screwsman in the country, hmm? Well, here's a fitting challenge!"
Agar: "Challenge? Seventy five seconds for two keys! It's nigh impossible!"
Pierce: "What if a snakesman comes over the roof and cracks the drum from the inside?"
Agar: "What snakesman's to break that drum? Nobody good enough is out. The best is Clean Willy, and he's in."