early 15c., recommendacion, "action of commending oneself to another" (a sense now obsolete), from Old French recommendation (Modern French recommandation), from Medieval Latin recommendationem (nominative recommendatio), noun of action from past-participle stem of recommendare (see recommend). Meaning "act of representing in a favorable manner, act of recommending (someone or something) as worthy" is from 1570s. Letter of recommendation is from c. 1500.
Letter of recommendation, a letter given by one person to another, and addressed to a third or "to whom it may concern," in which the bearer is represented as worthy of consideration and confidence. [Century Dictionary, 1895]