It was first originated from the Mighty Paladin whom is known as Serno. (the blood elf Protection Paladin) Paladinness refers to how much of a Paladin the person is and pretty much sums up all of the Paladins skills and abilities into one word.
"That guys Paladinness is a turn on"
"He has so much Paladinness!"
"I wish i had that much Paladinness..."