(found while trying to translate "en l'occurence")

http://www.thefreedictionary.com/nonce

the present or particular occasion

[From Middle English for the nones, for the occasion, alteration of for then anes : for, for; see for + then, neuter dative sing. of the; see the1 + ones, anes, once; see once.]

http://www.worldwidewords.org/qa/qa-non1.htm

It's recorded right back into medieval times but was originally created by mistake.

http://www.wordreference.com/enfr/nonce

• nonce: pour l'instant (pour le moment)
• for the nonce: pour la circonstance