A false shirt front or collar worn to look formal without wearing a full shirt, or a small donkey (British).
For the shirt meaning: likely from the nickname 'Dick' for Richard, with the suffix '-ie.' For the donkey meaning: from Dickie as a diminutive form. The origins are unclear but may relate to working animals.
A dickie is basically 19th-century clothing hack—people would wear just a fancy front panel to look dressed up while wearing their regular shirt underneath, kind of like the original fake-it-till-you-make-it fashion.
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