This is usually used as a joke when talking to friends. The penis is normal in shape untill one notices the end, where it has dramatically bulged into a mushroom shape. To get a mushroom cock one can use a lacky band so that the circulation builds up and the head swells. This person usually needs therapy or has a hot date.
Dewy: Sup bro whats been happening?
Jero: Nothing much. Got a huge pain in my penis, it's throbbing like hell. Lisa wont want to see me tomorrow.
Dewy: Hahaha, mushroom cock syndrome? Take that lacky band off the shaft so it doesnt throb you idiot.
Jero: It's not the lacky band, i took that off ages ago. This is prob from the STD i got from the rank chick that thought i had a Watermelon cock
Dewy: Fuck!! Dude seriously you gotta stop worrying about the shape of your cock it isn't healthy.
Jero: Yer i know but the hoes love it.
Dewy: No they dont.