Heaping Pelion upon Ossa. Adding difficulty to difficulty; fruitless efforts. The allusion is to the attempt of the giants to scale heaven by piling Mount Ossa upon Mount Pelion.
“Ter sunt conati imponere Pelio Ossam”.
Virgil Georgies i. 281.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894