E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Pelion.
Heaping Ossa upon Pelion. Adding difficulty to difficulty, embarrassment to embarrassment, etc. When the giants tried to scale heaven, they placed Mount Ossa upon Mount Pelion for a scaling ladder.
1
Ter sunt conti imponre Plio Ossam
Virgil: Georgics, i. 281.
A noteworthy hexameter verse. The i of conati does not elide, nor yet the o of Pelio.