A gadget, mechanical device, or unnamed thing whose name you can't remember—often used humorously for strange contraptions or complicated equipment.
American slang from the early 1900s, possibly derived from Spanish 'gizmo' or from circus/military jargon. The exact origin is disputed, but it became popular in the 20th century as informal American English for 'thingamajig' or 'whatchamacallit'.
Gismo is one of those wonderful words that entered English when we stopped having names for everything—it perfectly captures the spirit of the 20th century when new inventions appeared faster than we could name them!
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