This
phrase is an Idiom (a phrase or word that does NOT imply or talk about LITERAL meaning) that means ‘not fair or unjust’.
Meaning, an act or move that aggrieves, hurt or pained and even saddened
another person. ‘Below’ represents ‘under’.
‘the belt’
here resembles ‘boundary or limits’ that
have been ignored or forgotten. We are talking more about connotations of each
word not its LITERAL meaning or sense.
