Old English belt "belt; girdle; broad, flat strip or strap of material used to encircle the waist," from Proto-Germanic *baltjaz (source also of Old High German balz, Old Norse balti, Swedish bälte), an early Germanic borrowing from Latin balteus "girdle, sword belt," said by Varro to be an Etruscan word.
Transferred sense of "broad stripe encircling something with its ends joined" is from 1660s; that of "broad strip or tract" of any sort, without notion of encircling (as in Bible belt is by 1808). As a mark of rank or distinction, mid-14c.; references to boxing championship belts date from 1812. Mechanical sense is from 1795. Below the belt "unfair" (1889) is from pugilism. To get something under (one's) belt was originally literal, to get it into one's stomach (1839), figurative use by 1931. To tighten (one's) belt "endure privation" is from 1887.