Past tense of 'fess,' meaning to confess or admit guilt, often used in the phrase 'fess up.'
Shortened form of 'confess,' where the 'con-' prefix was dropped over time in informal American English. The word became popular slang by the early 20th century.
This is a great example of how language evolves through shortcuts—'fessed up' became so common that we stopped saying the full word 'confessed,' and now many people don't even realize it came from that original word.
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