1. A kingfisher
2. A mythical bird, identified with the kingfisher, that was fables to nest at sea about the time of the winter solstice and to calm the waves during incubation
1. Calm; quiet; peaceful; undisturbed; happy (e.g. "deep, halcyon repose")
2. Marked by peace and prosperity (e.g. "halcyon years")
This word dates to the 1540's in the phrase halcyon dayes. It comes from Latin and/or Greek, but it wasn't adopted as much as it was sort of translated. The Latin phrase is alcyonei dies and the Greek is alkyonides hemerai, they are both variants (or misspellings) of alkyon, which is a combination of hals + kyon ("sea, salt" + "conceiving") that means "kingfisher". Kyon is the present participle of kyein ("to conceive", literally means "to swell") which derives from the Proto-Indo-European base *keue- ("to swell"). Halcyone was the mythological daughter of Aeolus who threw herself into the sea and became a kingfisher after she was widowed.
Etymologies come from the Oxford English Dictionary and/or Etymonline.com
*Photo credit: PetcareGT.com