Now I know it's a slang word but what exactly is it's meaning, also is it eulbba or eul-ppa?
A straight off definition of 얼빠 means 'a person who likes someone or advocates someone based solely upon their looks'. So it implies that whoever is an 얼빠 means he or she judges people based on looks.
99% of the time the word is used to describe someone who when looking for a boyfriend/girlfriend/spouse, looks are the only standard that matters to them. So obviously, you have to be extra careful when using it because you can give off a shallow impression of yourself.
To help you semantically understand how 얼빠 means the above, you can break the word as such
얼빠 = 얼굴 (face) + 빠 (a slang that means an advocate of something)