From the scholia to Aristophanes, Birds 307, where the headword appears. (In the TLG Coulon & van Daele text it is spelled pipi/zousi, though contrast LSJ's pippi/zw, as here, and followed by Dunbar). Hesychius has almost the same gloss for the infinitive pipi/zein.
The reference to 'to water' is the verb pipi/skw, which is similar to pipi/zw and occasionally conflated with it. See e.g. the scholia to Aristophanes, Thesmophoriazusae 393, discussing the word oi)nopi/phs (an alternative to the accepted text's oi)nopo/tis: "Symmachus says this is from pipi/zein ['to give to drink'] wine; but I do not see pipi/zein in this meaning as attested in Attic, but rather spoken in imitation of the [bird] call." (LSJ derives it from neither, but instead from o)pipteu/w 'to spy on', and glosses the noun as 'gaping after wine'.) The verb is also used in the sense 'to suck' in beta 287.