The phrase "rubber thumb" refers to someone who is really good at typing or texting on their phone or computer, as if their thumb is flexible like rubber. Full definition
The face of the steering wheel is home to the chrome and rubber thumb controls for the audio system, which look nice enough, but have a mushy, vague feel. (cnet.com)
As a bonus, it comes with eight rubber thumb grips for your controllers. (amazon.com)
This Flea Comb is available in one size and features two rubber thumb supports, bamboo construction and an ergonomic handle. (petadventuresworldwide.com)