Arithmetic
Middle English: from Old French arismetique, based on Latin arithmetica, from Greek arithmētikē (tekhnē) ‘(art) of counting’, from arithmos ‘number’.
wiktionary
From Middle English arsmetike, from Old French arismetique, from Latin arithmētica, arithmeticus, from Ancient Greek ἀριθμητική ( τέχνη)(arithmētikḗ (tékhnē), “(art of) counting”), feminine of ἀριθμητικός(arithmētikós, “arithmetical”), from ἀριθμός(arithmós, “number, counting”), from Proto-Indo-European *h₂ri-dʰh₁-mó-s, form of *h₂rey-(“to count, reason”). Used in English since 13th century.
etymonline
arithmetic (n.)
"art of computation, the most elementary branch of mathematics," mid-13c., arsmetike, from Old French arsmetique (12c.), from Latin arithmetica, from Greek arithmetike (tekhnē) "(the) counting (art)," fem. of arithmetikos "of or for reckoning, arithmetical," from arithmos "number, counting, amount," from PIE *erei-dhmo-, suffixed variant form of root *re- "to reason, count."
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."
