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.
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."
What treat can we have now? If we were to treat ourselves now—that is, to have dainties a little above our means, it would be selfish and wicked. It is the very little more that we allow ourselves beyond what the actual poor can get at that makes what I call a treat .... But now—what I mean by the word—we never do make much of ourselves. None but the poor can do it. I do not mean the veriest poor of all, but persons as we were, just above poverty. [Lamb, "Old China"]