This is a bael or bael fruit AKA Bengal quince, wood apple, stone apple or seer phael (head-fruit). In Thai it’s known as matum.
In Thailand, bael is usually found in the form of dried slices, which are reconstituted in water to make juice. Our housekeeper brought over a few from her tree and I was surprised at how hard and heavy they were. We did as she said and boiled them, but then accidentally left them out on a hot night and the next day, they had fermented in the shell and burst, oozing a heavy syrup onto our counters. I threw them into the pond out front as an offering to Shiva, although he seems to favor the leaves instead of the fruit.