One of the mysteries of the English language finally explained.
Designating an object (usually an aerial or signal decoder) which is designed to be positioned on top of a television set. Frequently in "set-top box"noun Broadcasting a box-shaped signal decoder for viewing cable or satellite television or giving television access to the Internet, or one which converts a digital television signal to analogue for viewing on a conventional set.
1970s; earliest use found in B.S.I. News. From set + top.
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