The two parts of this entry are unrelated, except for being derived from the same adjective.
[1] Comparative degree (masculine nominative singular) of the commonplace adjective keno/s; cf. kappa 1316, kappa 1322. For its use in a proverb see gamma 491, tau 1217. Parallels in Choeroboscus, Dionysius Thrax, and the Etymologicum Magnum suggest that the glosses may have been intended rather as additional illustrations of the comparative formation.
[2] The "voider" is of course a purgative; so glossed in LSJ. This is the first attestation of the term, but it also occurs in Nicetas Choniates (CE 12-13), Historia III 1, p. 497 van Dieten: "[Alexius] sent doctors away with insults, saying that the only thing they would ever turn to was purgatives (kaqa/rsia), and that out of all the substances that work cures, the only thing they knew of and would pour in, was 'voiders'."