Gascon sanja, sansha, santja, sòntja, shansha, sancha, Aragonese sancha and dialectal Catalan sanxa (Cerdanya) designate a milking vessel carved in wood, often made by hollowing out a tree trunk1. On the other hand, dialectal Basque xantxa (L, Z), xaiñtxa (Z) designates a milking vessel with a long metal handle (kopetxa), and in the latter also 'ladle' (golhare), a meaning which in my opinion would be the original one2.
Although the Gaulish origin of this word seems undeniable, a Celtic etymology is more dubious. For example, Matasović proposes a Celtic femenine *sϕanjā corresponding to the masculine *sϕenjo- > Old Irish sine 'teat'3, in turn derived from IE *spen-, which is phonetically unacceptable, among other things because Celtic *sϕ- gives s- in Goidelic but f- in Britonic and probably also in Gaulish. For the same reason, Hubschmid's protoform *sand-ikā from IE *spn̥dh- 'bucket'4 can't be accepted, although Celtic *sϕondā would be the origin of Romansh s(u)onna 'bucket', probably a Lepontic loanword.
Following Coromines5, I'd propose a Gaulish protoform *sant-ikā as a Baltoid loanword corresponding to Lithuanian sámtis 'spoon, ladle', sémti 'to pump, to scoop (a liquid)', from an IE root *semH- 'to pump, to scoop (a liquid)' also found in Latin sentīna 'bilge; sewer, drainage' and cognate to Altaic *ʃŏ́mo 'to dive; to summerge; to scoop (a liquid)' (EDAL 2193) > Turkic *tʃo:(m)- 'to diver; to swim; to scoop (a liquid); to immerse, to dip' (Turkish čömče 'spoon'6). On the other hand, Etruscan śanti (Tabula Capuana) probably designated some kind of vessel.
1 G. Rohlfs (1970): Le Gascon. Etudes de phylologie pyrénéenne, § 59. It's likely the same kind of vessel called kaiku in Basque.
2 The oldest European metal ladlers are from the Hallstatt culture of the Early Iron Age.
3 R. Matasović (2009): Etymological Dictionary of Proto-Celtic, p. 333.
4 J. Hubschmid (1951): Alpenwörter. Ursprungs und vorromanischen Romanischen, p. 61.
5 DECLC, p. 667-668.
6 J. Hubschmid (1955): Schläuche und Fasser, p. 107, quotes čamča, čumča with a diminutive suffix -ča.