Ultimately from PCelt. *kladimo- or *kladiwo-. There are claims and counterclaims regarding possible borrowing between Celtic languages. According to RM, "the Brittonic words (MW cledyf, MBret. clezeff, Co. clethe) are early loanwords from Goidelic". Conversely, MDV gives the direction of borrowing as follows: OIr. claideb <- W. cleddyf < *kladimo-.
Michiel de Vaan. Etymological Dictionary of Latin and the other Italic Languages. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
Matasović, Ranko. Etymological Dictionary of Proto-Celtic. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online: http://iedo.brillonline.nl.ludwig.lub.lu.se/dictionaries/content/proto-celtic/index.html
Cornish Language Partnership online dictionary http://www.cornishdictionary.org.uk/
Robert Farren, PIE culture words collection, 2017
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