DIAGNOSIS A wobbling chuck usually means it's not seated properly on the spindle or the internal jaws are damaged/worn.
FIX First, try to reseat it. The chuck is held on by a screw with left-hand threads (tightens clockwise, loosens counterclockwise). Insert a long Allen key into the chuck, hold the chuck body, and give the Allen key a firm clockwise turn to ensure the retaining screw is tight.
IF THAT DIDN'T WORK The wobble might be in the spindle bearing or the gear assembly. This requires opening the drill. If the spindle itself wobbles, the tool needs professional repair.
HEADS UP A wobbling chuck will make every hole oversized and uneven. Don't use it for precision work until fixed. Replacement chucks are standard 3/8" keyless types and are relatively easy to swap if the spindle is okay.