The phrase "master thief" refers to a person who is extremely skilled at stealing things. They can steal valuable items without getting caught easily because they are experts at what they do. Full definition
Is somebody trying to follow in the footsteps of The Raven, the legendary master thief who disappeared years before? (shortpause.com)
Thief is the latest in the recently resurrected steampunk stealth franchise following the adventures of master thief Garrett as he sneaks through a semi-industrialized medieval city. (apgnation.com)
Thief: Deadly Shadows (# 1.99): A sequel may well be in development, but until then you may want to relive the antics of master thief Garrett with this classic title. (dealspwn.com)