One of three blades on lawn mower very loose and spinning freely
I was eventually able to get the right sized wrench 15/16th, and was able to back off the bold holding on the blade, then cleaned it up, and put it back on.  I had to put a block (2x4 chunk) to keep the other blades from spinning as I tightened it, but eventually got it tight enough that all seems to be back in working condition.

I'm still confused how it could get so loose so fast without hitting something, and also, if it was that loose, why was the bold so darn tight.  But alas, it was just the bolt that needed tightened.  Problem solved.