The cobbler should stick to his last

Is it an idiom:

The cobbler should stick to his last

Thank you.

Stick to what you know. Do what you do best. I assume you know that a cobbler's last is "a form (as of metal or plastic) which is shaped like the human foot and over which a shoe is shaped or repaired" (from the MWOD). The fellow who shapes or repairs the shoe is a cobbler, of course.
SS