[ganeshkrishnan]

It's incorrect translation. The correct one is "trouser friends" because they are so close they can borrow each other's shorts/trousers

[OJFord]

Haven't heard that one. 'Blood brothers' might be close. (In case you're not familiar, it's an ironic phrase, doesn't mean that the 'brothers' are related at all, just very close: https://www.urbandictionary.com/define.php?term=Blood%20Brot...)

Or, I suppose, 'BFF', but perhaps not even teenage girls can bring themselves to use that seriously today (I suspect it's 'so last year').