Thievish

A secret language of display coloration and hand signals. Thieves guilds, merchants, and the more underbelly elements of society use this language to set marks, coordinate crimes, and generally remain anonymous while speaking.

Thievish is included in the Gether Guide to Languages on DrivethruRPG:
https://www.drivethrurpg.com/product/328473/Gether-Guide-To-Languages