Plural of 'ef,' the name of the letter 'F' in English, or a variant spelling of 'effs' referring to euphemistic use of the word.
From Middle English ef, the name given to the letter F. Simple noun pluralization of the letter name, following English conventions for letter names.
Calling someone an 'ef' became safer slang for a worse insult—language users created a clever system where letter names become coded insults, which is why 'eff' entered dictionaries as polite fiction.
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