means To deal with or behave toward someone or something in a particular way, to handle a topic, to provide medical care, or to give someone an enjoyable thing — and as a noun, that enjoyable thing itself.

from From Old French 'traitier' (to deal with, negotiate, handle), itself from Latin 'tractare' (to handle, manage, drag about), a frequentative of 'trahere' (to pull or draw) — the same deep root that gives us 'tractor,' 'traction,' and 'tract.' So at its core, to 'treat' someone was to handle or pull them into your dealings; the modern sense of a pleasant gift or indulgence grew later from the idea of 'treating' someone well — entertaining them, standing them a round, footing the bill.