means the nausea, dizziness, and queasiness caused by the rocking motion of a boat or ship, a form of motion sickness triggered at sea.

from A plain compound of "sea" and "sickness," both rooted in Old English (sǣ and sēocnes). The word simply names what sailors have always known — that the heaving deck makes you ill. Greek had its own version: nautia, "ship-sickness," from naus (ship), which is also the ancestor of our word "nausea." So the very feeling of wanting to be sick is, etymologically, the feeling of being on a boat.