= (Something trivial) makes even a 100-year-old love turn cold/sour (in an instant).
In Japan, people commonly use this expression in a jocular and sarcastic manner to refer to the short-lived nature of most relationships that could come to an unexpected, abrupt end anytime instead of dwindling away over time, when, for instance, you happen to see your partner fly into an uncharacteristic rage over the most trivial of matters – in stark contrast to their usual demeanour – which you find off-putting to the extent that it would make even a 100-year-old love turn cold/sour in an instant..., jolting you from your lovely daydream back into cold reality.
This expression is strictly reserved for relationship contexts: The burning passion of love could all too easily die down at the sight of a small flaw you noticed that you wish you had remained blissfully unaware of. It might also be caused by a temporary lapse in the physical attractiveness of your partner. Anything seemingly trivial could turn out to be a trigger point!