Green’s Dictionary of Slang

button n.2

1. [mid-18C–late 19C] any form of illicit decoy; esp. a confederate of those running a game of three-card monte n. or thimble-rig n. [? fig. use of SE button, something small and worthless].

2. [1940s+] (US Und.) a lookout [? button man under button n.4 ].