Covered with a smooth, shiny coating like glass or a sugar coating; or looking dull and expressionless because of tiredness or lack of interest.
From 'glaze,' which comes from 'glass' through Old English and Germanic roots. The word originally meant to fit with glass, then expanded to mean any glass-like shiny coating.
Your eyes get 'glazed' when your brain stops processing what you're seeing—it's literally a medical term for when pupils stay fixed, which is why bored people and sleepy people can look similar.
Complete word intelligence in one call. Free tier — 50 lookups/day.