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halcyon (adj.) 1540s, in halcyon dayes (Latin alcyonei dies , Greek alkyonides hemerai ), 14 days of calm weather at the winter solstice, when a mythical bird (identified with the kingfisher) was said to breed in a nest floating on calm seas. From halcyon (n.), late 14c., from Latin halcyon , from Greek halkyon , variant (perhaps a misspelling) of alkyon "kingfisher," from hals "sea, salt" (see halo- ) + kyon "conceiving," prp. of kyein "to conceive," literally "to swell," from PIE root *keue- "to swell." Identified in mythology with Halcyone , daughter of Aeolus, who when widowed threw herself into the sea and became a kingfisher. <p style="text-align:right;color:#A8A8A8"></p>
Online Etymology Dictionary