[sc. A proverbial phrase] in reference to those doing something undeservedly.
For at the Mysteries the necessary items used to be conveyed out of town to Eleusis on donkeys. Hence the proverb, [used] because the donkeys bearing their burdens had an especially bad time of it. So since [Xanthias] would be experiencing the same things as [a donkey] suffering from the laden weight, Aristophanes adduces the proverb.