Origin and history of retain

c. 1300, reine, "strap of a bridle," attached to it on either side of the head, by which the rider or driver restrains and guides the animal, from Old French rene, resne "reins, bridle strap, laces" (Modern French rêne), probably from Vulgar Latin *retina "a bond, check," a back-formation from Latin retinere "hold back" (see retain). Compare Latin retinaculum "a tether, halter, rein."

The figurative extension of reins to "guidance, means of controlling; control, check, restraint" is by mid-14c. Hence many expressions, originally from horse-management: Hold the reins "wield power" (early 15c.); take the reins "assume the power of guidance or government" (1610s). To give something free rein also is originally of horses; to give (a horse) the reins (1620s) is to allow it free motion.

[fee to secure services] mid-15c., "act of keeping for oneself, an authorized retention (of dues, etc.)," an agent noun from retain (v.), or perhaps from or influenced by French retenir, infinitive used as a noun. Meaning "a retaining fee, fee paid to an attorney or barrister to secure his services" is from 1818. The general sense of "sum paid to secure special services" is from 1859.