What is the etymology of the informal usage of 'the deal' in phrases like "what's the deal with __?" or in "what's his deal?"
About the word 'deal', etymonline mentions
Business sense of "transaction, bargain" is 1837, originally slang, from the older sense of "arrangement among a number of persons for mutual advantage."
Big deal is from 1928 as "important transaction;" ironic use first recorded 1951 in "Catcher in the Rye."
But only 'big deal' feels related here.
The origin of this usage is interesting to me because it's one of the few ways to informally refer to the whole of a situation or the whole of a persons motivations and situation.