Verb
could have sworn
(informal) Is very sure about; especially when the thing in question is apparently not the case or could be doubted.
"You lost your bracelet?" "Yeah, but I could have sworn it was in the kitchen on the table when I last saw it."
Although I'm a retired teenager, I remember what it was like to be one. I could have sworn I was riding an emotional roller coaster most of the time. Looking back, I'm actually amazed that I survived. Barely. Sean Covey