Ultracrepidarian

MEANING:

adjective: Giving opinions beyond one’s area of expertise.
noun: One who gives opinions beyond one’s area of expertise.

ETYMOLOGY:

From Latin ultra (beyond) + crepidarius (shoemaker), from crepida (sandal). Earliest documented use: 1819.

NOTES:

The story goes that in ancient Greece there was a renowned painter named Apelles who used to display his paintings and hide behind them to listen to the comments. Once a cobbler pointed out that the sole of the shoe was not painted correctly. Apelles fixed it and encouraged by this the cobbler began offering comments about other parts of the painting. At this point the painter cut him off with “Ne sutor ultra crepidam” meaning “Shoemaker, not above the sandal” or one should stick to one’s area of expertise.
Addition: The story was told by the Roman writer Pliny the Elder, hence Latin. 

I have heard this sentiment phrased as "Cobbler, stick to your last."

Via A.Word.A.Day