immediately, at once. The Old English an-on or an-ane = at once. Variants, on one, anone.
“They knewye hym in brekyng of brede, and onone he vanyste awaye fro hem.” —MS. Lincoln, A 1, 17.
Spek the lion ... To the fox anone his wille.
Wright's Political Songs.
“For the nonce” is a corrupt form of “For the-n once,” where the-n is the accusative case, meaning “For the once” or “For this once.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894