The headword phrase, first attested in Pindar Pythians 2.69 (web address 1 below) with the noun implicit, is probably quoted from Plutarch, Lycurgus 22.2 (cf. Moralia 1140C): when the Spartans are about to join battle, a piper plays this. See further, LSJ s.v. *kasto/reios at web address 2. The present gloss is therefore a horrid misunderstanding of it, brought about by the ambiguity of both of its elements (kasto/reios = 'Castor's' or 'a beaver's'; me/los = 'song' or 'limb').
Adler cites an entry in the lexicon of Laurentianus 59.16 as a comparandum; not in other lexica.