Yours, Etc.: Origins and Uses of 8 Sign-Offs
Old English isig; see ice (n.) + -y (2). Modern use is said to be a late Middle English re-formation. Figurative use from 1590s. Related: Icily; iciness.
Excellent; good; cool: The old Valley Girl terms ''rad'' and ''icy'' still describe the very cool (1980s+ Teenagers)