1680s, "pertaining to a person's (physical or mental) constitution," from constitution + -al (1). Meaning "beneficial to bodily constitution" is from 1750. Meaning "authorized or allowed by the political constitution" is from 1765. Constitutional monarchy is recorded from 1801, from French. Related: Constitutionally.
"a constitutional walk," 1829, probably originally among university students, and probably short for constitutional walk or exercise; from constitutional (adj.) in the "beneficial to bodily health" sense.