No certain etymology. According to Houaiss, borrowed from Germanic either directly or through Fr. garagnon <- franconian wrainjo 'stallion'. But compare Ir. gearrán, ScG. gearran, Eng. garron, MIr. gearrán, possibly derived from OIr. gerr, gearr adj. 'short; castrated' and as substantive 'a short horse, a gelding (?)' (EDIL).
Houaiss, Antônio & MS Villar. 2001. Dicionário Houaiss da língua portuguesa. Rio de Janeiro: Instituto António Houaiss.
Simpson, J., & Weiner, E. S. 1989. Oxford English dictionary. Oxford: Clarendon Press.
Ó Dónaill, Niall & De Bhaldraithe, Tomás (red.) (1977). Foclóir Gaeilge-Béarla. Baile Átha Cliath: An Rún.
Robert Farren, PIE culture words collection, 2017
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