οὐκ ἂν λάβοις παρὰ τοῦ μὴ ἔχοντος → you can't take from one who doesn't have, you can't squeeze blood out of a turnip, you can't get blood out of a turnip, you can't get blood from a stone, you can't get blood out of a stone
Pettălus: i, m.,
I the mythic name of a hero who sought to attack Perseus at the court of Cepheus, Ov. M. 5, 115.