means A soft, glossy brightness on a surface, where light reflects smoothly rather than harshly.

from From Old English 'sciene,' meaning beautiful or bright — a cousin of German 'schön' (beautiful). For centuries 'sheen' was an adjective meaning 'fair' or 'radiant,' the kind of word that adorned knights and maidens in poetry. Only later did it slip into use as a noun, the visible shine itself, possibly pulled along by the rhyming gravity of 'shine,' to which it's related through the same old Germanic root for light and showing.