So basically, when you tell someone that they're a panna, you're telling them they're too old to be a virgin still.
Often used as an insult between a group of friends--usually male--to make the virgins of the group to feel embarrased. The proper, and non-partiers of the the group are generally the ones targetted for this slur.
So basically, the if you call someone a panna, you're a total douche-bag.
ALSO: If telling a single someone that they're a 'panna', you'd call them a 'panee'...which is the third-person singular indicative present tense noun of 'panna'.
OTHER FORMS: panee, pannas, panner, pannly
Ludwig: "You're revolting."
Heracles: "Don't be like that man! You're just jealous I got 3 hott babes to sleep with me last night!"
Ludwig: "N-no, I'm not! And anyways, I could get a girl if I wanted to!"
Heracles: "Hey man, don't take your sexual frustration out on me just 'cause your a panna."
Ludwig: "I'M NOT A SEXUALLY FRUSTRATED PANNA!" D:<
Feliciano: "What's a panna?"
Heracles: *facepalm* "We've got one too many pannas in our group..."