below-the-belt

The phrase originates from the sport of boxing, where a fundamental rule prohibits striking an opponent "below the belt" (i.e., on or below the waistline), as such blows are considered unsportsmanlike and dangerous. By the early 20th century, this literal prohibition was metaphorically extended into general discourse. The "logic" of the metaphor is direct: an unfair verbal attack is equated with an illegal physical blow, both targeting a vulnerable area outside the accepted rules of engagement. Thus, the phrase evolved from a specific athletic regulation into a common idiom describing any underhanded or excessively personal criticism.

This is a fixed adjectival/adverbial phrase. It is not productively formed from separable morphemes for analysis in the traditional sense. Its meaning is idiomatic and derived from the rules of boxing.