When I was little dad would ask ‘Daijoubu desu ka?’, which is Japanese for ‘Are you OK?’ In context, dad was asking ‘OK?’ I heard dad and others shortening the phrase to ‘Daijobe?’ It seemed to be fairly common amongst Navy people in San Diego at the time.
How widespread was ‘Daijobe?’ as American slang for ‘OK?’ forty years ago?
When I retired in 1990, it was still being used quite commonly for Navy folk who had spent any time in Japan. I still use certain words picked up on my two deployments there, along with some Vietnamese words and a couple of German words.
“Daijoubu” is just shortened, spoken between two people that know each other fairly well. It’s like saying in English “You okay?/You alright?” as opposed to lengthening it to the formal “Are you okay?” I’d imagine that it’d be just as popular 40 years ago as it is today since it’s more of an informality than a slang, but that’s just my guess.