drinking glasses with flat bottoms and no handle or stem, or acrobatic performers who do somersaults and flips.
From 'tumble' (to roll or fall) + '-er' (one who does). Originally named for glasses that would tumble if not carefully set, or performers who tumble through the air.
Medieval tumblers were acrobats, jugglers, and clowns who traveled between villages entertaining people before modern entertainment existed—they're the ancestors of circus performers and parkour athletes.
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