People who kill or destroy something, or in modern slang, people who do something really well or impressively.
From 'slay,' Old English 'slægan' meaning 'to strike' or 'kill.' The -er suffix makes it 'one who slays.' Modern slang use emerged from hip-hop culture in the 1990s.
The word 'slay' completely changed meaning in recent years—it went from 'to kill in combat' to 'to absolutely crush it,' which shows how fast young people reshape language.
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