late 15c., "self-controlled; kept under control," past-participle adjective from restrain. Related: Restrainedly.
mid-14c., refreinen, transitive, "exercise control over, restrain; hold (someone or something) back from action," senses now obsolete, also "exercise control over" (thoughts, desires, feelings, vices, etc.); from Old French refraigner, refrener, refreiner "restrain, repress, keep in check" (12c., Modern French réfréner).
This is from Latin refrenare "to bridle, hold in with a bit, check, curb, keep down, control," from re- "back" (see re-) + frenare "restrain, furnish with a bridle," from frenum "a bridle," a word of uncertain etymology (de Vaan supports a theory that it is connected to fretus "relying on").
The classical spelling was restored in French but not in English. In Middle English chiefly transitive. Intransitive sense of "forbear, keep oneself (from)" is from mid-15c. Reflexive sense of "control oneself, put restraint upon oneself" is from late 14c. Related: Refrained; refraining.
"having control of one's mental faculties, self-possessed," c. 1600, past-participle adjective from collect (v.). Related: Collectedness.