Etymology

This n. is best derived from an ON *skiu, which continues the original nom. sg. form *skiwi, and comes down to OIcel as the (rare) by-form skí ‘cloud’; cp. the more common OIcel variant ský (Far ský, Norw, Sw, Dan sky) which represents the oblique cases (< *skiuja-), and which > ME ski(e), PDE sky, and in WGmc cp. OE scēo (attested only once possibly meaning cloud, see further Dance), OS skio (neut., < PGmc *skewa(n)). 

PGmc Ancestor

*skiwi

Proposed ON Etymon (OIcel representative)

skí ‘cloud’ (variant ský)
(ONP ský (sb.))

Other Scandinavian Reflexes

Far ský, skýggj, Icel ský, Norw sky, Dan sky, Sw sky

OE Cognate

scēo ?'cloud'

Phonological and morphological markers

absence of palatalization of */sk/

Summary category

A1*c

Attestation

Attested by MED from the beginning of the 13c., this variant is mainly SWM and N (inc. various alliterative poems), in contrast with the more widespread skī(e n.

Occurrences in the Gersum Corpus

Cl 483, 1206, 1759 etc. WA 561, 4865

For a probable further instance, see skwez (n.).

Bibliography

MED skeu (n.1) , OED skew (n.1) , HTOED , HTOED , Dance skwez, Bj. 115, de Vries ský, Mag. ský, Bj-L. sky (1), Orel *skiwōn, AEW scīo