mid-13c., from Old French contenance "demeanor, bearing, conduct," from Latin continentia "restraint, abstemiousness, moderation," literally "way one contains oneself," from continentem, present participle of continere (see contain). Meaning evolving Middle English from "appearance" to "facial expression betraying a state of mind," to "face" itself (late 14c.).
late 15c., "to behave or act," from countenance (n.). Sense of "to favor, patronize" is from 1560s, from notion of "to look upon with sanction or smiles." Related: Countenanced; countenancing.