A PERSON or People who discriminate or social prejudice against people with disabilities. It can also be someone who judges or makes fun of someone with a disability or handicap.
Example: Someone who makes fun/discriminates someone in a wheelchair or someone with down syndrome.
Abby - "People in wheelchairs are retarded."
Sandy - "Wow you are such an ableist."
Abby is an ableist because she said people in wheelchairs are retarded. Not true.
Oh hello! Didn't see you there, I understand that you got called by some 14 y/o girl on Twitter "ableist". Don't know what that means? I got ya fam, "ableism" is a discrimination against people with disabilities. You didn't do any of that? Too bad, we know but they don't understand. Don't worry about it.
"John: Today, disabilities are used as excuses..."
"✨Emily✨: OMG you're such an ableist!!! Hope you get bullied!"