quay: [14] Quay is of Celtic origin. Its immediate source was Old French kai, but this was borrowed from Gaulish caio, which went back to an Old Celtic *kagio-. The spelling quay was introduced from modern French in the 17th century. The homophonic cay ‘small coral island’ [18] comes from cayo, a Spanish borrowing from French quai. => cay
quay (n.)
1690s, variant of Middle English key, keye, caye "wharf" (c. 1300; mid-13c. in place names), from Old North French cai (Old French chai, 12c., Modern French quai) "sand bank," from Gaulish caium (5c.), from Old Celtic *kagio- "to encompass, enclose" (cognates: Welsh cae "fence, hedge," Cornish ke "hedge"), from PIE *kagh- "to catch, seize; wickerwork, fence" (see hedge (n.)). Spelling altered in English by influence of French quai.