Klein wins

[lethargic_man]

My Oxford English Dictionary, Oxford Dictionary of English Etymology, and Collins Concise dictionary all merely list "bitumen" as having derived from Latin bitūmen. Rather to my surprise, I was able to get some more detail from Klein's Etymological Dictionary of the Hebrew Language (whilst looking up בֶּטוֹן beton, "concrete", which turns out to derive, via French, from this word):

L. bitūmen (= mineral pitch), an Osco-Umbrian loan word (the genuine Latin form form would have been *vetūmen, from *gwetūmen, of Celtic origin.

Cool, eh?

(Still doesn't tell me where *gwetūmen came from, though: it looks too long to simply be a single term meaning "bitumen, pitch". What do the individual parts of it mean?)