Why is the ninth month called September?
1710, "carrying out calculations," present participle adjective from calculate. Meaning "shrewdly or selfishly seeking advantage" is attested from c.1810.
1560s, "to compute, to estimate by mathematical means," from Latin calculatus, past participle of calculare "to reckon, compute," from calculus (see calculus). Meaning "to plan, devise" is from 1650s. Replaced earlier calculen (mid-14c.), from Old French calculer. Related: Calculable.