cobbler let the cobbler stick to his last people should only concern themselves with things they know something about (the cobbler's last is a shoemaker's model for shaping or repairing a shoe or boot). The saying, recorded in English from the mid 16th century, is a translation of Latin ne sutor ultra crepidam, attributed by Pliny originally to the Greek artist Apelles (fl. 325 bc) when a cobbler criticized not just his representation of a shoe, but also the rest of the picture.
the cobbler to his last and the gunner to his linstock a fanciful extension of let the cobbler stick to his last. The gunner's linstock was a long pole used to hold a match for firing a cannon. The saying is recorded from the mid 18th century.

cob·bler / ˈkäblər/ • n. 1. a person who mends shoes as a job.2. an iced drink made with wine or sherry, sugar, and lemon.3. a fruit pie with a rich crust on top.

cobbler
1. Sweetened cold drink made from fruit with wine or liqueur and ice.

2. Meat or fruit dishes topped with scone rounds.

cobbler XIII. of unkn. orig.
Hence cobble² vb. mend roughly, patch up XV.