"I'm a doctor, not an.." escalator, bricklayer, mechanic, coal miner, etc.
"I'm a doctor, not a" was Dr. Leonard McCoy's catchphrase on Star Trek: The Original Series and was later used in tribute by Dr. Julian Bashir on Deep Space 9, The Doctor on Star Trek: Voyager, Dr. Phlox on Star Trek: Enterprise, and by a variety of other characters outside the medical profession.
From Memory Alpha:
Very often, it was just the way of Dr. McCoy and subsequent characters to prevent criticism in case of failure, but also often, the "not a…" ability was successful, especially in dire needs.
The line has been misquoted in media as "Damn it, Jim, I'm a doctor…" although McCoy never used this expletive in the series. However this version of the line was eventually used by the alternate reality Leonard McCoy.
