Retard

"Retard" is a slur that was used from the 1400's to the 1900's as a genuine slur to discriminate against those with mental health issues such as autism or ADHD. In the 2000's its meaning has shifted to be synonymous with stupid and is no longer targeted towards those with mental health issues. Many people on the internet will call you an asshole for it, some will support you for it, use at your own risk.

TLDR; Originally a slur, now synonymous for stupid.

Person 1: That guy is stupid.

Person 2: Yeah he's retarded.