Etymology

femto-

Today it means: In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁵ (short scale quadrillionth or long scale billiardth). Full definition →

Danish femten ← Old Norse fimmtán ← Proto-Germanic *fimftehun
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The story

  1. Proto-Indo-European*pénkʷe
  2. Proto-Germanic*fimf
  3. Proto-Indo-European*déḱm̥
  4. Proto-Germanic*tehun
  5. Proto-Germanic*fimftehun
  6. Old Norsefimmtán
  7. Danishfemten
  8. English todayfemto-

From Danish femten (“fifteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains atto-.

Etymology from Wiktionary, CC BY-SA 4.0.