Means crazy, wild, or shockingly intense, whether it’s a scrap, a near miss, or a night out that went sideways. Said with the right tone it’s half disbelief, half admiration. You’ll hear That was mental after anything that feels off the scale. Just watch it when talking about real mental health stuff.
"That coaster launched us backwards in the dark and my stomach tried to leave my body. Proper mental, mate. Fancy a chippy after?"