[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.
[one kept in service] 1530s, "dependent or follower of a person of rank or position," agent noun from retain (v.). Also used in the general sense of "one who or that which retains or holds" (1540s). Meaning "dental structure used to hold a bridge in place" is recorded from 1887.