Backtalk is a rude reply to someone who is telling you what to do — a child to a parent, a student to a teacher, a worker to a boss. The “back” here is the talking-back kind: you push the order back at the person instead of taking it. It is informal and disapproving. You will hear it more in American English; British speakers often say backchat for the same idea. It can be a verb too, especially in American speech: “Don’t backtalk me.” The close cousin is the phrasal verb talk back. A normal question (“Why?”) is not automatically backtalk; the sting is in the tone — cheek, challenge, a refusal to be told. Though actually, what counts as backtalk depends on who is in charge: the same sentence can be honest curiosity at the dinner table and backtalk in a strict classroom.