Swear blind
Definition 1 of 1
Pro Tip 1/3Strong Assurance
Used when someone guarantees truth with high conviction, despite doubts from others. 
He'll swear blind the deal is safe, but I'd check the details first.
Pro Tip 2/3Emotional Emphasis
Emphasizes how strongly a person feels about what they're saying. 
She swore blind that it was the best decision for everyone involved.
Pro Tip 3/3Informal Speech
Primarily used in casual conversation, not in formal writing or speech. 
In court, avoid phrases like 'swear blind'; use 'I assure you' instead.