"Mere Bagatelle"

Google provided this:

>>>In a prose passage of "The Sea and the Mirror" (1944), a character observes that "if the intrusion of the real has disoncerted and incommoded the poetic, that is a mere bagatelle compared to the damage which the poetic would inflict if it ever succeeded in intruding upon the real." This admonition is a leitmotif in Auden's work."<<< (Auden was alluding to utopianism.)

I remember my mother in the 40's grimly but airily dismissing inconvenient plumbing bills or broken china as "mere bagatelles".

I have a feeling that it is Smart Society slang dating from the Roaring Twenties, implying "I have so much money that this expense is not worth noticing."