To publicly support or approve of something or someone; to write your name on the back of a check to cash it.
From Old French 'endosser' (to put on the back), from 'en-' (on) + 'dos' (back). The financial meaning came first, and the approval meaning developed later.
When a celebrity endorses a product, they're literally putting their name and reputation 'on the back' of it—a metaphor that comes directly from signing the back of old checks.
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