Etymology

that is to say

Today it means: In other words. Used to introduce a clarification, simplification, or explanation Full definition →

French c'est-à-dire
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The story

  1. French · calque ofc'est-à-dire
  2. English todaythat is to say

Calque of French c'est-à-dire.

Etymology from Wiktionary, CC BY-SA 4.0.