A marble made of glass, used in children's games; or informally, something made of glass.
From 'glass' + '-ie/-y' (informal diminutive suffix). Common in children's marble games and playground vocabulary.
In schoolyard marble games, a 'glassie' was worth more points than regular clay marbles because glass marbles were rarer, shinier, and would crack your other marbles if you weren't careful!
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