1. A seal is basically a girl/guy (or even an item) that you can't have in the sense that she/he/it can never be yours.
Being a seal is relative since your seal probably hooks with people (who you probably hate for this fact) and is therefore not a seal to them.
This definition of seal comes from the person's likeness to the animal: the fact that they're so cute/hot/desirable that you want them but you also simultaneously want to club over the head (like with baby seals) because you can never have them.
2. Also, this can be used as a suffix or modifier to names to disguise the fact that you are talking about someone in the same room. Usually that person is a seal by the above definition as well before 'seal' is used to modify their name.
Guy 1 (frustrated): "Man! There are so many seals in this bar tonight!!"
Guy 2: "Yeah I know... I just wanna club them over the head so no one else can have them..."
Guy 1: "Who's that cute girl by the bar?"
Guy 2: " Hmm I don't know her name but she's definitely a baby seal."
Elsa = Anagram Seal (because the name is an anagram of the word seal)