Italian (Tuscan) bischero 'peg (on a string instrument); fool, idiot' derives from Etruscan viscri 'penis', attested in a votive inscription: eti viscri ture Arnθ Alitle Pumpuś (TLE 685) '... gives this penis', engraved on a figure of a man offering his male organ found at Paterno di Vallombrosa (Arezzo).
Although many specialists have linked the Etruscan word to Latin vīscus, vīsceris 'entrails' (itself a word of problematic etymology), I think the latter would correspond to Etruscan visc 'center' (see here). In my opinion, viscri would be an Italic loanword akin to Latin virga 'twig, rod'1 < *wizg-ā, from a lexeme *wizg- also attested in Germanic *wiskō 'bundle, besom'. The semantic shift 'rod' > 'penis' also happened in Spanish verga < Latin virga.
1 With rhotacism.