Past tense of 'cap,' meaning to put a limit on something or to cover with a cap or lid.
From Old English cæppe meaning 'hood' or 'covering,' borrowed from Late Latin cappa. The sense of 'putting a limit on' developed from the idea of putting a cover or top on something, first recorded in the 16th century.
The phrase 'to cap it all' meaning 'to complete or crown an achievement' comes from the academic tradition of placing a cap on graduates' heads. This ceremonial capping has been practiced since medieval universities, making graduation caps one of the oldest surviving academic traditions.
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