Gaulish buððutton 'spindle; penis' (updated)

Gaulish buððutton 'spindle; penis'¹ is attested on the spindle whorl Gallo-Latin inscription Moni gnatha gabi buððutton imon 'Come, girl, take my penis'². The word must have designated the instrument itself and then applied to the male organ in a metaphoric way. Basque buztan 'tail; penis' is presumably a Celtic loanword, probably from Gaulish itself.

From this and other Insular Celtic words (Old Irish bot 'penis, tail', Middle Welsh both 'umbo, shield boss'), Matasović reconstructs a Celtic protoform *buzdo- 'tail'³, supposedly derived from IE *gwozdo- (Germanic *kwast(j)ō 'bunch of branches', Albanian gjeth 'leaf, foliage', Slavic *xvost 'tail'⁴), assuming the original meaning was 'to sprout'.

On the other hand, English button is a loanword from Old French boton (modern bouton) 'bud; button; pimple, spot', itself from Late Latin *buttōne-, usualy regarded as a Germanic borrowing, but IMHO actually from Gaulish, which would be also the source of Germanic *buddōn 'bud'.

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¹ X. Delamarre (2008): Dictionnaire de la langue gauloise, p. 92-93. 
² W. Meid (1994): Gaulish Inscriptions, translates the Gaulish word as 'kiss', cfr. bussu- 'lip'. 

³ R. Matasović (2009): Etymological dictionary of Proto-Celtic, p. 85-86.
⁴ Conflated by Vassmer to *gvozdis 'nail'.