Originally Posted by FBinNY

If, as suggested above, you mean the crank, the only real solution is a new crank.

Once a crank is ridden loose for any distance, the square hole wears in a way that allows movement as you pedal. That movement may be slight at first, but will worsen over time. The movement will work the bolt loose, allowing more movement. It's a progressive spiral that once started cannot be stopped.

Fortunately, the damage is limited to the crank, since the spindle is so much harder. So a new crank, and a reminder to keep it tight, and retighten immediately at the first hint of looseness.