Dried nasal mucus; dried snot from your nose.
From Dutch 'bogel' or possibly Afrikaans origins, the word entered English slang in the 20th century. It's informal and often considered juvenile, though universally understood and anatomically accurate.
Kids have used booger as a gross-out word forever, but it's actually just a normal body function—your nose produces mucus to filter air and keep you healthy! The word probably came from Dutch settlers.
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