bouncing breasts
乳揺れ あばれる乳 おっぱいぷるんぷるん 荒ぶるおっぱい アニぱい 乳揺れ(うごイラ) 乳分身 乳摇
Breasts in motion, usually in an animation. If it is not an animation, something like a motion blur or motion lines must be present to convey a sense of movement.
If the breasts are bouncing in opposite directions, add the unaligned breasts tag as well.