I’ll throw them out; they’re on their last knees

We do seem to mix body parts in phrases, don’t we?  This one was uttered when doing some spring cleaning.  It is a mix of “on its knees” (in a weakened or desperate state) and “on its last legs” (about to stop working).  And make sure you throw out those jeans with holes in the knees!  A big thanks to Kerry Reynolds who heard this one and passed it on!