arithmetic (n.) Look up arithmetic at Dictionary.com
"art of computation, the most elementary branch of mathematics," 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" (source also of Old English, Old High German rim "number;" Old Irish rim "number," dorimu "I count;" Latin ritus "religious custom;" see read).

The form arsmetrik was based on folk-etymology derivation from Medieval Latin ars metrica; the spelling was corrected early 16c. in English (though arsmetry is attested from 1590s) and French. The native formation in Old English was tælcræft, literally "tell-craft."