A cute nickname for someone you have affection for. Normally this nickname is given by an older guy to a younger girl, of which he either is with or fancies. However, the use of this normally leads to the girl thinking of the guy as a big brother, rather than dating material.
"Hiya Pup!" Man, she looks good "Hey. I just got a new boyfriend, I think he's wonderfullllllllll. I want you to meet him cause you're so special to me."
"...Sure, I'd love too." Stupid Nickname