idiom · touching a surface very lightly and momentarily, similar to a dragonfly skimming water

他在她的额头上蜻蜓点水般地吻了一下。

idiom · to touch on something lightly without going into it deeply; performing a task in a cursory or perfunctory manner

调查研究不能蜻蜓点水，必须深入基层了解实情。