"well!" French, literally "let us go," first person plural imperative of aller "to go."
Examples from the Web for allons
It is a little slatternly—it is a good deal stained—it isn't becoming—it smells of cigar-smoke; but, allons donc!Roundabout Papers|William Makepeace Thackeray
Allons, there is no road yet, but we are all Aarons with rods of our own.Fantasia of the Unconscious|D. H. Lawrence
“Allons—dites lui les noms de toute la garniture,” said Madame de Fontanges to her attendants.Newton Forster|Captain Frederick Marryat
There the others joined in: "Allons donc pour un litre—let's drink a litre in the canteen."In the Foreign Legion|Erwin Rosen
"Allons, mes enfants, table—and the one who has no appetite shall be cast into the sea—by the heels," added his reverence.A Village of Vagabonds|F. Berkeley Smith