To rattle, wobble, or be bumpy/unstable (used for furniture, structures, vehicles, etc.).
机の脚が一本短くて、書いているときにがたつく。
One leg of the desk is short, so it wobbles when I write.
古い家なので、歩くたびに床ががたつく音がする。
Because the house is old, the floor creaks and rattles every time you walk.
未舗装の道を走ると車ががたついて疲れる。
Driving on an unpaved road makes the car shake and is tiring.