The headword (taken from the Aristophanic line quoted) is the aorist imperative of the rare verb ἐγκανάσσω ; see also Euripides, Cyclops 152. It implies a large quantity quickly poured (and drunk).
[1] Glosses from the scholia vetera to the Aristophanic line about to be quoted. cf. Hesychius, Lexicon epsilon188, the same headword glossed with some of these same terms: ἐγκάναξον: ἔγχεε, ἔκπιε . See also Photius, Lexicon (Theodoridis, 1998) epsilon36: ἐγκανάξαι: ἐγχέαι οἶνον .
[2] cf. the same scholion: “ἐγκάναξον , from a reed basket [κανοῦν ]. But some apply it to a throng, from a sharp sound [καναχή ], that is, pour it in noisily.”. In the same sense, and with similar words, see also Etymologicum Magnum 310.1.
[3] Aristophanes, Knights 105.