According to my Apple Dictionary it means 1- a thing of little importance; a very easy task. 2- a game in which small balls are hit and then allowed to roll down a sloping board on which there are holes, each numbered with the score achieved if a ball goes into it, with pins acting as obstructions. 3 - a short, light piece of music, esp. one for the piano.
Synonyms include there weren't any listed. I'll have a go myself. 1 - trifle. 2- n/a 3- impromptu (?)
Remarks. If Bagatelle was a person they would definitely be male, on the far side of 50 wearing a rather threadbare white shirt, open to the navel, with more nicotine-yellow hair framed by that slightly discoloured placket than remains on the upper quadrant of his head, which is otherwise cloaked in a once-blonde prince valiant-type do. Gold watch (9 kt) and lots of pistol gestures that would begin to look slightly despairing after 11.30pm and three too many Black Russians. Would probably live on a decrepit boat with disintegrating bamboo blinds. No pets. They'd tie him down, man.