Celtic *longā 'boat,
vessel' is attested in Welsh llong 'ship', Old Breton locou
'vessels, vases', Old Irish long 'vessel, (little) vase, ship', to
which corresponds the Gaulish toponymic element Longo-1. There's also Cisalpine Gaulish
(Todi) lokan /longan/ 'cinerary urn', an accusative form where /ng/
is rendered as k.
Although some authors have suggested a loanword (with reanalysis) from
Latin nauis longa 'warship' (lit. 'long ship'), specialists such as
Matasović2 think this is a genuine Celtic word
without IE etymology, which I consider it to be a cultural loanword from North East Caucasian *leqˀV 'a k.
of vessel' (NCED 1511), where the ejective stop became
prenasalized. Also related would be Latin lanx 'dish, plate'3, apprently borrowed from Etruscan in account of its /a/ vocalism.
This lexeme was also borrowed into IE *lonko-/ā 'valley' > Lithuanian lankà ‘valley, rivermeadow’, Old Church Slavonic lǫka ‘gulf, valley, meadow, marsh’, Tocharian B leṅke ‘valley’, and Late Latin *lanca ‘depression, bed of a river’4, with a straightforward semantic shift.
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1 X. Delamarre (2008): Dictionnaire
de la langue gauloise, p. 206-207.
2 R. Matasović (2009): Etymological
Dictionary of Proto-Celtic, p. 244.
3 This meaning is reflected in
Tsezian.
4 J.P. Mallory & D.Q. Adams (2006): The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World. 122.