Used curse words or profanity; or took a solemn oath to tell the truth.
From Old English 'swerian' meaning 'to swear.' The dual meaning developed because both curse words and oaths were originally spoken with intense feeling and spiritual weight.
Swear has meant both 'curse' and 'promise under oath' for 1,000 years—because both require you to call on something greater than yourself with absolute sincerity!
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