retainer (n.1) Look up retainer at Dictionary.com
"fee to secure services," mid-15c., "act of keeping for oneself" from retain (perhaps influenced by Middle French retenir, infinitive used as a noun). Meaning "fee paid to an attorney to secure his services" is from 1818.
retainer (n.2) Look up retainer at Dictionary.com
"servant," 1530s, agent noun from retain (v.). Meaning "dental structure used to hold a bridge in place" is recorded from 1887.