1590s, "pertaining to management of a household," perhaps shortened from economical, or else from French économique or directly from Latin oeconomicus "of domestic economy," from Greek oikonomikos "practiced in the management of a household or family" (also the name of a treatise by Xenophon on the duties of domestic life), hence, "frugal, thrifty," from oikonomia "household management" (see economy (n.)). Meaning "relating to the science of economics" is from 1835 and now is the main sense, economical retaining the older one of "characterized by thrift."
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.