the.com/low blow

means An unfair criticism or action that exploits a vulnerable or personal weakness rather than engaging fairly with the real issue.

from Straight out of the boxing ring: a punch landed below the belt line, illegal under the rules of fair fighting. By the early-to-mid 1900s, 'low blow' had climbed out of the ring and into everyday speech, describing any unfair verbal or social gut-punch — one that dodges the rules of a fair fight and hits where it hurts instead.