(A). A trifle; a thing of no consideration. “Oh! nothing. A mere bagatelle.” In French, “Il dépense tout son argent en bagatelles” means, he squanders his money on trash. “Il ne s'amuse qu'à des bagatelles,” he finds no pleasure except in frivolities. Bagatelle! as an exclamation, means Nonsense! as “Vous dîtes qu'il me fera un procès. Bagatelle!” (fiddlesticks!)
“He considered his wife a bagatelle, to be shut up at pleasure” [ i.e. a toy to be put away at pleasure]. —The Depraved Husband.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Bagatelle from Infoplease: