Below the belt
Definition 1 of 2
Pro Tip 1/3Emotional Impact
Using 'below the belt' shows the action was hurtful, not just unfair. 
After the argument, his 'below the belt' remarks left her in tears.
Pro Tip 2/3Informal Usage
Considered informal, best used in casual conversations or familiar settings. 
Hey, no teasing about weight—that’s below the belt.
Pro Tip 3/3Avoid in Professional Contexts
In work settings, this phrase might be too casual or accusatory. 
I refrained from saying his critique was below the belt during the meeting.