It means "very stupid" but in a more hilarious, weird, and/or spontaneous way. From the Latin word for "slow".
If you want to tick off a politically correct person, just say it. They think it's mean because
people with mental problems are not always stupid, except you can'
t call those
people "retarded" in the first place, and nobody cares anyway. Plus, once again, it's from the Latin word for "slow".