The headword is the Attic spelling preference for this masculine noun, here in the nominative singular; see LSJ s.v. ou)do/s. (In the quotation given it appears in the accusative singular.)
[1] The gloss is the same grammatical form as the headword, but with the alternative spelling; cf. omicron 826, omicron 287, and chi 193 gloss.
[2] Sophocles, Oedipus at Colonus 1590-1 (web address 1), with scholion; the messenger relates to the Chorus how Oedipus stopped to bathe before the underground portal. [Although the Suda transmits katafra/kthn (with a coat of mail, bandaged; see LSJ s.v. katafra/kths), it is apparent that katarra/kthn (down-rushing, sheer, abrupt; see LSJ s.v. katarra/kths) in the received texts is correct.]