treat (v.)
c.1300, "negotiate, bargain, deal with," from Old French traitier "deal with, act toward; set forth (in speech or writing)" (12c.), from Latin tractare "manage, handle, deal with, conduct oneself toward," originally "drag about, tug, haul, pull violently," frequentative of trahere (past participle tractus) "to pull, draw" (see tract (n.1)).

Meaning "to entertain with food and drink without expense to the recipient by way of compliment or kindness (or bribery)" is recorded from c.1500. Sense of "deal with, handle, or develop in speech or writing" (early 14c.) led to the use in medicine "to attempt to heal or cure, to manage in the application of remedies" (1781). Related: Treated; treating.
treat (n.)
late 14c., "action of discussing terms," from treat (v.). Sense of "a treating with food and drink, an entertainment given as a compliment or expression of regard" (1650s) was extended by 1770 to "anything that affords much pleasure."