Slang for an experienced bike mechanic.
It takes several years to become a shop rat. Only after spending thousands of hours building bikes, building wheels, rebuilding suspension forks and shocks, setting up derailleurs, replacing bottom brackets, rebuilding hubs, matching chainrings/cranksets/cassettes, and bleeding every type of hydraulic brake system does one become a shop rat.
A shop rat is the guy/gal you want to talk to about your bike's issues. They've seen every possible problem a million times over and know how to fix it professionally and quickly. They also know secret repair tricks that you'll never find in a service manual or on YouTube.
The opposite of a Tool Box Ted
I spent 10 years as a shop rat in a Colorado bike shop.
The main Pinkbike guys all used to be shop rats for something like 10+ years before their current jobs.