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mid-13c., arsmetike, from Old French arsmetique (12c.), from Latin arithmetica, from Greek arithmetike (tekhne) "(the) counting (art)," fem. of arithmetikos "of or for reckoning, arithmetical," from arithmos "number, counting, amount," from PIE root *re(i)- "to reason, count" (cf. Old English, Old High German rim "number;" Old Irish rim "number," dorimu "I count;" Latin ritus "religious custom;" see read).
Originally in English also arsmetrik, on folk etymology from Medieval Latin ars metrica; spelling corrected early 16c. Replaced native tælcræft, literally "tell-craft."