Note: This appears to be a variant form not in standard dictionaries; treating as plural of small fibers or diminutive fibrils.
From 'fibre' (British spelling) plus the diminutive suffix '-let' (from Old French '-let') meaning small.
While not standard, the term 'fibrelets' demonstrates how English speakers naturally create diminutive forms—just as we have 'piglet' and 'booklet,' we can imagine tiny fibers as 'fibrelets.'
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