A smaller number of; used with countable nouns to indicate a reduced quantity.
From Old English 'feawe' (few) + comparative suffix '-er'. Germanic root shared with German 'wenig'. The distinction between 'fewer' and 'less' developed as English became more standardized.
The 'fewer vs. less' grammar rule (fewer for countable things, less for uncountable) was actually invented by one man - Robert Baker in 1770 - and wasn't widely followed until the 20th century, showing how modern grammar 'rules' are often quite recent inventions.
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