I only started commonly seeing these berry-like fruits this year. They seem to be growing in popularity up here in Issan, but I suspect they were brought here from another part of Thailand, where they are apparently have shorter names: Nam daeng and nam phrong.
They are apparently used in India for pickles.
I had started calling these Lao Cherries, but there are a couple other fruits already called that (plus they don’t seem to be from this area), so I finally just looked it up.
So the important thing: Do they taste good?
They taste like vitamin C punched you in the throat. Like the sourest mango and unripe lemon (hence the name? Mamuang = mango; Manao = lemon/lime) in the world are frolicking on your tongue. So naturally, Thais eat them dipped in chili sugar and stupid farang stuff three in their mouth at a time to see if it can be done in a sort of personal stupidity challenge.
So wikicheatia has a long paragraph on names for this fruit which, in the spirit of university plagiarism, I will only only slighty modify before pasting here:
maha karamba (Sinhala)
kerenda (in Malaya)
Bengal currant (South India)
Christ’s thorn (South India)
nam phrom (Thailand)
Karja tenga (Assam)