Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. posca (pusca, Veg. Vet. 2, 48), ae, f. [perh. πόω, poto], an acidulous drink of vinegar and water, Plaut. Mil. 3, 2, 23; id. Truc. 2, 7, 48; Cels. 4, 5 fin.; Plin. 27, 4, 12, § 29; 28, 5, 14, § 56; Scrib. Comp. 46; Suet. Vit. 12; Spart. Hadr. 10; cf.: posca, ὀξύκρατον, πόσις, Gloss. Philox.
2. Posca, ae, m., a Roman surname: M. Pinarius Posca, Liv. 40, 18.