unfair; disregarding the rules
1941-46; after illegal punch in boxing
Old English belt "belt, girdle," from Proto-Germanic *baltjaz (cf. 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.
As a mark of rank or distinction, mid-14c.; references to boxing championship belts date from 1812. Mechanical sense is from 1795. Transferred sense of "broad stripe encircling something" is from 1660s. Below the belt "unfair" (1889) is from pugilism. To get something under (one's) belt is to get it into one's stomach. To tighten (one's) belt "endure privation" is from 1887.
early 14c., "to fasten or gird with a belt," from belt (n.). Meaning "to thrash as with a belt" is 1640s; general sense of "to hit, thrash" is attested from 1838. Colloquial meaning "to sing or speak vigorously" is from 1949. Related: Belted; belting. Hence (from the "thrash with a belt" sense) the noun meaning "a blow or stroke" (1899).
Nasty; malicious; dirty: That remark was below the belt/ his below-the-belt tricks
Not according to decent and usual practice; unconscionably irregular: to hit below the belt/ aim below the belt
[1890+; fr boxing, where blows to the lower part of the body are forbidden]