Girls (or guys, I guess) who can't get over themselves! They beg for attention and take pictures, often showing an excess cleavage or sticking out their ass. The plural to "slug" is "sluggys". Sluggys are often rather stuck-up and obsessed with themselves, and getting a girlfriend or boyfriend. When they do, in fact, have that boyfriend or girlfriend, they tend to say "I love you" too quickly and end up getting dumped/breaking up with them just as quickly. Sluggys are not liked. A synonym to slug is "slore".
Person1- "Oh my gosh did you SEE her tumblr?"
Person2- "I know, she's such a slug."
Person1- "You're like a coin!"
Slug- "Aw, valuable?!"
Person1- "Nah, two-faced."