salt
n.— «With that, Madrid became what’s called “a salt”: a union employee who infiltrates nonunion work sites. His work is part of a larger effort by New Orleans trade unions to combat what they call an underground cash-only economy fueled by the post-Hurricane Katrina construction boom.» —“Migrant workers fleeced, union says” by Katy Reckdahl in New Orleans, Louisiana Houston Chronicle (Texas) Aug. 18, 2007. (source: Double-Tongued Dictionary)