A constitution is a statement of the basic principles and laws of a nation, state, or group, such as the U.S. Constitution. Another very common meaning of constitution is the physical makeup of a person.
The noun constitution is from Latin, from constitutus, "set up, established," plus the suffix -ion, meaning "act, state, or condition." So think of a constitution as how a body (yours, the government's) is set up. If you have a strong constitution, it means you don't get sick very often.