A grammatical contraction of the words "killer" and "sweet" used to emphasize the positive characteristics and connotations of each respective word. Any thing, act or indivual referred to as "Killa Sweet" must be really really cool. There are no true synonyms for "Killa Sweet", because there is no other phrase in the English language that is able to communicate such an intense level of awesomeness. That is why almost every use of "Killa Sweet" ends in an exclamation point. Anything else would not be able to fully get the message across.
This word originated in South-Eastern Pennsylvania circa 2006.