Used when you're insisting something is 100% true, usually while everyone else is looking at you like you've lost the plot. It's that extra gear beyond I swear, where you’re putting your name on it even if you’re wrong. Comes out a lot in little everyday dramas like missing keys, missed calls, or who ate the last biscuit. No one’s actually going blind, it’s just big talk.
"I swear blind I put the keys on the kitchen side. Checked everywhere. Only thing there’s a mouldy banana and your Tesco Clubcard. So who’s nicked them?"