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

I understood it to be an insult rather than a neutral "stick to what you do best", more like "don't get above yourself". I've noticed on google that there is a persistant explanation that the origin was the artist Apelles (C4th BC) who said this to a cobbler who found fault with the shoes in a painting. Apelles corrected the mistake. The cobbler went on to criticize the shape of the legs when Apelles said "Cobbler, stick to your last!" This may just be an oft-repeated google story, though. Pamela