"pertaining to a diet in the 'assembly' sense," 1845; see diet (n.2) + -al (1).
Entries linking to dietal
diet n.2
"assembly of delegates, etc., held from day to day for legislative, political, or other business," mid-15c., from Medieval Latin dieta, variant of diaeta "daily office (of the Church), daily duty, assembly, meeting of counselors," from Greek diaita "regimen" (see diet (n.1)), but associated with Latin dies "day" (from PIE root *dyeu- "to shine"). Since c. 1600 used by English and French writers of the legislative assemblies of Germany and Austria.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).