Arguably "years later" isn't instant.

Karma came from a culture that believes in reincarnation. It is payback for your behavior in past lives. "Instant karma" is therefore payback for your behavior in your current lifetime, without an intervening turn of the samsara cycle.

If you kicked a puppy right before you died, and got slapped too hard just as you were born into your next life, that's regular karma. If you stole someone's car for a joyride when you were 16, and your car was stolen by a joyrider when you were 90, that's instant karma. The number of intervening years is not the important factor, but the number of intervening rebirths.

First, thanks. I honestly had never heard that description before. I definitely see it making more sense than the alternative interpretation of the phrase if you approach the concept of Karma as an actual metaphysical phenomenon rather than as more of a meme, as I think people often do.

That said, any time I've seen the phrase "instant karma" used, it's in response to something that occurs within seconds or minutes of what earned it, or "instant" in a more commonly used sense.

It seems that there are various interpretations of karma. Dogen, founder of the Soto Zen school, taught that karma can be experienced in this same life. I swear I've read far earlier sources discuss karma in a manner outside of rebirth in samsara, but I can't find those sources at this time.

I can't help you on the religious origins of this, but the concept got introduced to me by a game by this dude named Richard Garfield in the early 90s. If I recall correctly, the term that that dude and a bunch of other WotC folks used was "mana".

It took a while before I learned that it was apparently a term in hawaiian slang, but it usually seems appropriate, especially when you're discussing this with folks old enough to remember alpha, beta, and unlimited Magic: the Gathering.