The expression “to pile (or heap) Pelion on Ossa” means: to heap difficulty upon difficulty; to attempt that which is all but impossible.
Pelion is a mountain peak in Thessaly, about 5,300 feet high; Ossa, another peak in Thesssaly, about 6,500 feet high.
The story is told in Homer’s Odyssey (Book XI) as follows, according to the Butcher and Lang translation:
And after her I beheld Iphimedeia, and she bare children twain, but short of life were they, godlike Otus and far-famed Ephialtes. Now these were the tallest men that earth, the grain giver, ever reared, and far the goodliest after the renowned Orion. At nine seasons old they were of breadth nine cubits, and nine fathoms in height. They it was who threatened to raise even against the immortals in Olympus the din of stormy war. They strove to pile Ossa on Olympus, and on Ossa Pelion that there might be a pathway to the sky. Yea, and they would have accomplished it, had they reached the full measure of manhood. But the son of Zeus destroyed the twain, ere the down had bloomed beneath their temples, and darkened their chins with the blossom of youth.