Origins
The English word "countenance," used both as a noun and a verb, traces its origins to the Latin verb "continēre," meaning "to hold together" or "to restrain." This Latin verb itself is composed of the prefix "con-" meaning "together," and the root "tenēre," meaning "to hold." The root "tenēre" is inherited from the Proto-Indo-European root *ten-, which carries the general sense of "to stretch" or "to hold." The semantic development of "countenance" is closely tied to the notion of holding oneself together, which manifests in the outward expression or demeanor of a person.
The noun "countenance," referring to a person's face or facial expression, as well as their composure or self-control, entered English in the 13th century. It was borrowed from Old French "contenance," which itself derived from the Latin noun "continentia." The Latin "continentia" means "self-restraint" or "composure," and is a noun formed from the present participle stem of "continēre." Thus, the Old French "contenance" carried the sense of demeanor, bearing, or composure, which was then adopted into Middle English as "countenance."
The semantic evolution from the Latin "continentia" to Old French "contenance" and then to English "countenance" reflects a shift from the abstract notion of self-restraint or holding oneself together internally, to the external manifestation of that internal state as visible in the face or bearing. In other words, one's "countenance" is literally how one "holds oneself together," which is outwardly observable in facial expression and demeanor.
Latin Roots
The verb form of "countenance," meaning "to admit as acceptable," "to tolerate," or "to approve," developed later in English. This verbal sense is metaphorically connected to the original meaning of maintaining composure or self-restraint in the face of something. To "countenance" an action or behavior is to hold oneself together in the presence of it, thereby implying tolerance or approval. This verbal usage is not directly inherited from Latin as a verb but is rather a semantic extension within English, based on the noun's connotations of composure and bearing.
It is important to distinguish that the English "countenance" is not a direct inherited cognate from Latin in the sense of continuous development within a single language lineage but rather a borrowing from Old French, which itself was derived from Latin. The Old French "contenance" and Latin "continentia" share the same root elements but represent different stages in the word's history. The Proto-Indo-European root *ten- is the ultimate source of the Latin "tenēre," and thus of "countenance," but this root is not attested directly in English outside of inherited cognates from Latin or Romance languages.
"countenance" as a noun entered English in the 13th century from Old French "contenance," which came from Latin "continentia," itself derived from "continēre" ("to hold together"). The components "con-" ("together") and "tenēre" ("to hold") reflect the Proto-Indo-European root *ten- ("to stretch, to hold"). The word's meaning evolved from the abstract idea of self-restraint or composure to the visible expression of that composure in the face or demeanor. The verb sense of "countenance" in English, meaning to tolerate or approve, is a later development based on the noun's semantic field rather than a direct Latin verbal inheritance.