From the second edition (1989):
halcyon, n. and a.
(ˈhælsɪən, ˈhælʃɪən) Forms: 4 alceon, alicion, 6 alcion, halsion, 6–7 halcion, 7 alcian, 6– alcyon, halcyon. [a. L. halcyon, more properly alcyon, a. Gr. ἀλκυών kingfisher.
The spelling ἁλ- hal-, is supposed to have arisen out of the fancy that the word was f. ἅλ-ς sea + κύων conceiving, connected with the fable that the halcyon broods upon her nest floating on the calm sea in the ‘halcyon days’.]

A. n.

1. A bird of which the ancients fabled that it bred about the time of the winter solstice in a nest floating on the sea, and that it charmed the wind and waves so that the sea was specially calm during the period: usually identified with a species of kingfisher, hence a poetic name of this bird.

1390 Gower Conf. II. 106 (Bodl. MS. 294) Hir briddes ȝit‥Of Alceon þe name bere. 1398 Trevisa Barth. De P.R. xix. lxxix. (1495) 910 In the cliffe of a ponde of Occean, Alicion, a see foule, in wynter maketh her neste and layeth egges in vii dayes and sittyth on brood‥seuen dayes. 1545 Joye Exp. Dan. Ep. Ded. (R.), Thei saye, that in the‥coldest tyme of the yere, these halcions (making their nestis in the sea rockis or sandis) wille sitte their egges and hatche forth their chickens. c1592 Marlowe Jew of Malta i. i, How stands the wind? Into what corner peers my halcyon's bill? a1631 Drayton Noah's Flood (R.), There came the halcyon, whom the sea obeys, When she her nest upon the water lays. c1750 Shenstone Elegies v. 22 So smiles the surface of the treach'rous main As o'er its waves the peaceful halcyons play. 1819 J. H. Wiffen Aonian Hours (1820) 104 The brilliant halcyons‥fluttering upon azure wings, appear Loveliest above secluded waters. 1867 Contemp. Rev. VI. 252 The alcyon sits her floating nest.
fig. a1649 Drummond of Hawthornden Poems Wks. (1711) 39/1 Makes Scotland's name to fly On halcyons wings‥Beyond the ocean to Columbus shores. 1880 Goldw. Smith in Atlantic Monthly No. 268. 200 The halcyons of literature, art, and science were floating on the calm and sunlit sea.

b. In Zool. a kingfisher of the Australasian genus Halcyon, or of the subfamily Halcyoninæ.

1772–84 Cook Voy. (1790) V. 1805 We found the halcyon, or great king-fisher, having fine bright colours. 1802 R. Brooke's Gazetteer (ed. 12) s.v. P. William's Sound, The birds found here were the halcyon, or great kingfisher [etc.].

2. Calm, quietude, halcyon days. Obs.

1647 Trapp Comm. Matt. ix. 15 Our halcyons here are but as marriage feasts, for continuance. 1654 —— Comm. Ps. ii. 4 By this means the Church had an happy Halcyon. 1748 Richardson Clarissa (1811) II. 4 'Tis well one of us does [want courting], else the man would having nothing but halcyon. 1797 A. M. Bennett Beggar Girl (1813) IV. 144 All, therefore, was halcyon with Mrs. Woudbe.

B. attrib. passing into adj.

1. Of, or pertaining to, the halcyon or kingfisher. halcyon days [Gr. ἀλκυονίδες ἡµέραι, L. alcyonei dies, alcyonides, alcedonia]: fourteen days of calm weather, anciently believed to occur about the winter solstice when the halcyon was brooding.
For the allusion in quot. 1605 see kingfisher.

[1540 R. Hyrde tr. Vives' Instr. Chr. Wom. (1592) Pj, Wherefore those daies be called in Latine Halcionii, that is as you would say, the Halcion birdes daies. 1545 Joye Exp. Dan. 2a (Stanf.), I remembred the halcyons dayes. 1591 Shakes. 1 Hen. VI, i. ii. 131 Expect Saint Martins Summer, Halcyons dayes.] 1601 Holland Pliny x. xxxii. (R.), They lay and sit about mid-winter‥and the time whiles they are broodie, is called the halcyon daies: for during that season the sea is calm and navigable, especially in the coast of Sicilie. 1605 Shakes. Lear ii. ii. 84 Bring oile to fire, snow to the colder moodes‥and turne their Halcion beakes With euery gale, and varry of their Masters. 1839 Penny Cycl. XIII. 230/1 The fable of the floating cradle in which during the Halcyon dayes the bird was said to rear its young.

2. Calm, quiet, peaceful, undisturbed. (Usually qualifying days.)

1578 Chr. Prayers in Priv. Prayers (1851) 464 It hath pleased thy grace to give us these Alcyon days, which yet we enjoy. 1631 Gouge God's Arrows v. xvii. 429 Were our daies more halcyon, more quiet and peaceable. 1641 Evelyn Mem. (1857) I. 12 Fortifications (a great rarity in that blessed halcyon time in England). 1665 Sir T. Herbert Trav. (1677) 11 When two are seen, they foretel Halcyon weather. 1841 D'Israeli Amen. Lit. (1867) 250 Peace and policy had diffused a halcyon calmness over the land. 1878 Masque Poets 218 The bird of love, in days so truly halcyon, Upon the billows well might build her nest.