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"fee to secure services," mid-15c., originally "act of keeping for oneself" from retain, or perhaps from or influenced by Middle French retenir, infinitive used as a noun. Meaning "fee paid to an attorney to secure his services" is from 1818.
"servant," 1530s, agent noun from retain (v.). Also "one who retains or holds" (1540s). Meaning "dental structure used to hold a bridge in place" is recorded from 1887.
retainer re·tain·er (rĭ-tā'nər)
One that retains, as a device, frame, or groove that restrains or guides, especially for a prosthesis.
An appliance used to hold teeth in position after orthodontic treatment.