its in the middle of an erection/hard on and your regular dick size. In Other Words, its as if your penis is indecisive about getting up. usually happens when a girl is grinding on you or any other "sexual dance"...
man: Whoa I popped a chubby when that girl showed me her tits .
friend: Lucky you because i jizzed myself . . .
when the penis becomes half erect. Not to the extent that it an erection, but it is larger than normal and almost there. This can be due to anything sexually stimulating, from talking about gay porn to not being able to get it up during sex.