The adjective impeccable describes something or someone without any flaws. A stand-up comedian needs impeccable timing for his jokes to work.
The adjective impeccable refers to something or someone without marking or error — but it can also mean to be spotless or clean. The word comes from the Latin impeccabilis and means "to be sinless," which is also one of the senses, now outdated, in English. You can see how "clean" comes from "sinless." Because of the messy nature of picnic food, it is highly unlikely that you would leave with your clothing still as impeccable as when you arrived, especially if you like mustard and ketchup!