commence (v.) Look up commence at Dictionary.com
c. 1300, from Old French comencier "to begin, start" (10c., Modern French commencer), from Vulgar Latin *cominitiare, originally "to initiate as priest, consecrate," from Latin com "with, together" (see com-) + initiare "to initiate," from initium "a beginning," noun use of neuter past participle of inire "to go into, begin," from in- "into, in" (from PIE root *en "in") + ire "to go" (from PIE root *ei- "to go").

Spelling with double -m- began in French and was established in English by 1500. Related: Commenced; commencing.