variant of in- before -b-, -m-, -p- in the sense of "not, opposite of" (immobile, impersonal; see in- (2)) as well as "in, into" (implant, impoverish; see in- (1)). In some English words it alternates with em- (1).
c. 1300, "moderate, modest, discreet" (a sense now obsolete), from Old French mesurable "restrained, moderate, sensible; restricted," from Late Latin mensurabilis, from mensurare "to measure," from Latin mensura "a measuring, a measurement; thing to measure by," from mensus, past participle of metiri "to measure," from PIE root *me- (2) "to measure." Meaning "that can be measured" is from mid-14c. Related: Measurably.