The definition of a contraption is a strange machine, appliance or device used to do something.(noun)
A strange little bike that you built yourself to get you from place to place that looks as if it is going to fall apart at any minute is an example of a contraption.
See contraption in Webster's New World College Dictionary
Origin: < ? con(trive) + trap, + -tion
See contraption in American Heritage Dictionary 4
Origin: Perhaps mock-Latinate blend of contrive
Origin: and trap1.
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