"pertaining to the rules for regulating the kind and quantity of food taken," 1570s, from Latin diaeteticus, from Greek diaitetikos "of or pertaining to diet," from diaita "way of life, regiment" (see diet (n.1)). Related: Dietical (1610s).
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.
<a href="https://www.etymonline.com/word/dietetics">Etymology of dietetics by etymonline</a>
Harper, D. (n.d.). Etymology of dietetics. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/dietetics