Someone who has a partner, girlfriend or significant other, but often engages in an emotional affair with another person. Whilst they engage emotionally with the person, the Spurdog avoids engaging in activities which could result in him/her being accused of cheating. Activities common to a Spurdog include snuggling, texting late at night and chatting via Instant Messaging applications such as Facebook chat.
It is a digression from the term "Shark" which tends to give the emotional cheat a feeling of power and elegance. Instead it refers to the Squalus genus of shark, a lowly bottom feeder with the colloquial name "spurdog".
I spy a spurdog.
He's not really gooing to sleep with her, he's just a spurdog