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Webster 1913 Edition

Noun.

The figurework, often fantastic and delicate, which moisture sometimes forms in freezing, as upon a window pane or a flagstone.

Noun.

Work resembling hoar-frost on shrubs.

frostwork ‎(countable and uncountable, plural frostworks)

Any naturally occurring intricate pattern of ice crystals
- Frostwork keeps its place on the window within three feet of the stove all day in my chamber. – Thoreau, H., Winter: From the Journal of Henry D. Thoreau, Houghton, Mifflin and Company, 1888, p 343.