Mostly used by people in their teen years to express their desperate need to get laid, often used together with pictures of "good looking people". Commonly used as a hashtag together with a selfie as well. The person usually pouts and tries to look sad.
"DUDE OMG HYUNA *picture of Hyuna* !!! # CAAH"
"dude that girl you like just posted a selfie with the hashtag #CAAH. You should totally go for it."