When the guys penis is so small that it doesn't break the girls hymen, thus causing her to still be a technical virgin.
Debbie: So how was it last night?
Susan: I'm a half-a-virgin now.
Debbie: Oh, I'm sorry.
The term half a virgin was used in the movie Mean Girls. You are a half virgin if you have given or recieved oral sex but have not had actual sex.
Regina: I was "half a virgin" when i met him
You become 'Half-A-Virgin'... or use the excuse, because you've tried VERY VERY VERY hard to remain pure. But have this one experience when a dick 'may or may not have' entered a place it wasn't supposed to.
Hence, you are 'half'. It was in there, but not long enough for it to take away your virginity.
Person K: So has anyone ever had a PIECE OF YOUR PIE?!
Person A: No. I'm Half-A-Virgin.
Person K: How is that possible?!
Person A: Maybe you should check out UrbanDictionary, to answer that question.
Someone who takes it up the front but not the back or the back and not the front
Lexa is half a virgin because she takes like 4 dicks up the booty but not the front