This is the 5759th most frequent Japanese word.
とんとん
"とんとん" can mean "knocking lightly," "tapping sound," or describe things being "even or balanced," depending on context.
In this context, 'とんとん' imitates the sound of light knocking, emphasizing the action of tapping or knocking gently.
とんとん叩いてドアを開けた。
Knocking lightly, I opened the door.
Here, 'とんとん拍子' (tonton byo-shi) suggests rapid and smooth progress akin to rhythmic actions similar to the sound 'とんとん'.
計画がとんとん拍子に進んでいる。
The plan is progressing smoothly and quickly.
The phrase 'とんとん返し' uses 'とんとん' to describe a sudden or surprising reversal, possibly imitating the abrupt nature of the sound.