ゆすぐ or すすぐ : rinse (the mouth)
I got in trouble the first time I went to dentist for this one. The dentist was like “Please ゆすぐ now” and I had no idea what to do.
Edit : I also remember being confused by another verb, basically to ask me to close the mouth and bite (to create a mold If I remember correctly), maybe 噛む ?