One's constitution was one's overall state of physical and mental health.
In 2267, Doctor Leonard McCoy suggested that even if Lazarus had the constitution and recuperative powers of a dinosaur, it still wouldn't explain how come his forehead wound disappeared. (TOS: "The Alternative Factor")
John Torres' mother warned him not to marry Miral, as she didn't think he had the constitution to live with a Klingon. He recalled this to his brother Carl in the 2350s, a comment young B'Elanna Torres overheard. (VOY: "Lineage")