optimum (n.)
1879, from Latin optimum, neuter singular of optimus "best" (used as a superlative of bonus "good"), probably related to ops "power, resources" (in which case the evolution is from "richest" to "the most esteemed," thus from PIE root *op- "to work") or to ob "in front of," with superlative suffix *-tumos. Originally in biology, in reference to "conditions most favorable" (for growth, etc.). As an adjective from 1885.