A tool or person that cuts something into slices, or in golf, a shot that curves sharply to the side.
From the verb 'slice' (of unknown origin, possibly from Old French) plus the agentive suffix '-er' meaning one who does something. The word became common in the 19th century as slicing became an important kitchen task.
In golf, a 'slicer' is actually a player with a technical flaw—the ball curves instead of flying straight—so the word that started as neutral (one who slices) became a mild insult among golfers.
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