A drinking glass with a flat bottom and no handle or stem, or a person who performs acrobatic flips and somersaults.
From 'tumble' (from Middle English 'tumelen,' related to Old Norse 'tumla'). Originally described a person who tumbles, then came to mean the glass, possibly because early tumblers rounded bottoms made them tumble when set down.
Medieval tumblers weren't called glasses—they had rounded bottoms that literally made them tumble and roll on wooden tables, which is how the glass got its name from the acrobatic performer!
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