Answer:
Depends what you mean:
The longest muscle in the body is sartorius, and it curls down the length of the thigh. It literally means 'to do with tailoring' and there are numerous theories as to how it got this name.
The muscle with the longest name in the body is called levator labii superioris alaeque nasi, and when translated from the Latin it describes its actual function "lifter of the upper lip and wing of the nose". So this muscle flares out your nose when you lift your upper lip in a 'snarl'!