1.smack in your face.
2.smack on your face
are both correct and why?
I'll assume you mean "smack" as a noun: "She will give you a smack...".
In this case "in your face" and "on your face" are both common in most contexts and I have no problem with either.. "on" is probably more usual.
As always with matters of usage, ideas on this will differ.
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