1917, in reference to branches of government, from inter- "between" + ministerial.
Entries linking to interministerial
inter-
word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of root *en "in."
A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.
ministerial adj.
1560s, in religion, "pertaining to the office, character, or habits of a clergyman;" 1650s, in politics, "of or pertaining to a minister or ministry of the state;" in some uses from French ministériel and directly from Medieval Latin ministerialis "pertaining to service, of a minister," from Latin ministerium "office, service, attendance, ministry," from minister "inferior, servant, priest's assistant" (see minister (n.)). In some cases probably directly from minister or ministry. Related: Ministerially.