"Dealio" is a neologism, with a meaningless "-io" attached to a real word, "deal." It is used as slang, most commonly in a sentence such as "What's the dealio?" "Deal" in this sense refers to specific situation. It could be some type of negotiation—in this case, the question would mean, what am I (or we) going to get, and what do we have to give up? Or it could refer to a general situation, meaning that the question would express "what's happening?" or "what are we going to do?"
Adding the ending, which makes it seem as if "deal" is derived from Latin or Greek (instead of Anglo-Saxon) likely gives the speaker a sense of faux sophistication. The suffix might also be a way for a speaker to signal informality, asking a serious question in a kind of lighthearted manner, as if they are not too anxious to know the answer.
English speakers sometimes add nonsensical endings, or endings borrowed from another language, to common words. For example, "-ito" and "-ita" are Spanish suffixes that signal something is small or young. If someone wants to indicate their home is tiny, they might say, jokingly, "my houseita." Typically such invented words do not become neologisms and enter the language, or if they do, it is as obscure slang.
"Dealio" is not in common English usage, though there are examples of it dating back at least two decades. In 2010, a singer named Missy Elliott recorded a song called "What's the Dealio," and the same line was used in a popular 2004 comedy movie, Napoleon Dynamite.