late 14c., trēte , "discussion, negotiation; agreement, contract, an accord," from Anglo-French treté , Old French traitié "assembly, agreement, dealings," from Latin tractatus "discussion, handling, management," from tractare "to handle, manage" (see treat (v.)). The sense of "contract or league between two or more nations or sovereigns solemnly ratified by the supreme power of each" is attested by early 15c. In Middle English also sometimes in now-obsolete sense of "treatise, discourse" (mid-14c.).
c. 1300, trēten (intrans.), "negotiate, debate or discuss for the purpose of settling a dispute;" late 14c. as "bargain, deal with;" from Old French traitier "deal with, act toward; set forth" in speech or writing (12c.). This is from Latin tractare "manage, handle, deal with, conduct oneself (in a certain manner) toward," literally "drag about, tug, haul, pull violently," frequentative of trahere (past participle tractus ) "to pull, draw" (see tract (n.1)). Compare entreat.
The sense of "deal with, handle, or develop in speech or writing" is from early 14c.; in reference to tangible objects, "deal with or touch physically," late 14c. The use in medicine "attempt to heal or cure, to manage in the application of remedies," is by early 15c. (Chauliac); one of the Middle English senses of treat (n.) was "medicinal salve" (late 14c.).
The meaning "entertain with food and drink without expense to the recipient by way of compliment, good will or kindness" is by 1710. Related: Treated ; treating .