When a promiscuous female (or homosexual male) has had semen ejaculated into their mouth/vagina/anus so many times by many different partners that the area begins to smell like old sperm.
"Mate I took Scarlet home last night..."
"Ouch! How'd it go?"
"Not good, she had a wicked case of cum hum..."
"Not good then?"
"Oh God no, I thought I was going to throw up on her and end up churning her butter!"
When a female (or homosexual male) has had so much sperm ejaculated inside of them (anus, mouth or vagina) that these areas actually smell like old sperm.
"Oh man, I took Scarlet home last night."
"How was it?"
"Terrible, she had a wicked case of cum hum."
"Oh man, that's horrific."
"Tell me about it, I almost threw up."