E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Taking a Sight.
Putting the right thumb to the nose and spreading the fingers out. This is done as much as to say, Do you see any green in my eye? Tell that to the marines; Credat Judæus, non ego. Captain Marryat tells us that some of the old coins of Denmark represent Thor with his thumb to his nose, and his four fingers extended in the air; and Panurge (says Rabelais, Pantagruel, book ii. 19) suddenly lifted his right hand, put his thumb to his nose, and spread his fingers straight out to express incredulity.
The eacristan he says no word that indicates a doubt,
But puts his thumb unto his nose, and spreads his fingers out.