[ I or T ] to make a series of quick delicate movements up and down or from side to side, or to cause something to do this:

[ I ] If your heart or stomach flutters, you feel slightly uncomfortable because you are excited or nervous:

[ C usually singular ] a short period of excited activity:

[ C usually singular ] UK informal a small bet (= money risked), especially on a horse race:

[ S ] a quick up-and-down movement