Used as slang to mean that you gave someone chlamydia or to explain that you have chlamydia. It is usually associated with an nsa hookup whereby the person that is 'clapped up' didn't realise the person they slept with had chlamydia.
--Can be used as clapped up or clapped 'subject' up--
"Hey man your hair looks like a pile of giraffe shit!"
"I know man my barber clapped me up!"
--
"Did you hear what Alex said about Izzy?"
"Yeah bro! A black dude overheard and clapped him up! He's in the hospitalnow..."