Uncertain. Possibly from a collective of Proto-Indo-European*udn-omn(“body of water”) reanalysed as a feminine singular and with metathesis of -dn- to -nd-, from *wódr̥, *udn-(“water”), attested in Italic as Umbrianudor(“water”).[1]
The resemblance to Proto-Germanic*unþī(“wave”) appears to be accidental, with at most minor semantic confluence.
Vulgar Latin: *undidiāre (see there for further descendants)
References
^ De Vaan, Michiel (2008) “unda”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 641
Further reading
“unda”, in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
“unda”, in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
unda in Gaffiot, Félix (1934) Dictionnaire illustré latin-français, Hachette.
Carl Meißner, Henry William Auden (1894) Latin Phrase-Book[1], London: Macmillan and Co.