Seasick means made ill by the movement of a boat. Get seasick, feel seasick, seasick the whole way.
It is motion sickness, caused by the inner ear reporting movement the eyes cannot confirm, which is why looking at the horizon helps and reading below does not. Most people adapt within a couple of days at sea. Note carsick and airsick, built the same way; and that Nelson is said to have been seasick throughout his career, which is the fact everybody offers as consolation.
1adjectiveseasickMade ill by the movement of a boat.
“The waves were so big that I felt seasick the moment we left the harbour.”
seasick is a state of motion sickness.
Middle English see seke is sea plus sick, and carsick and airsick were built later on the same pattern. English forms these freely and has done for six hundred years.
Middle English
see seke
English, today
seasick
From Middle English see seke, equivalent to sea + sick.
Etymology from Wiktionary, CC BY-SA
Practised in Sea & Sailing