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.
painting on a wall, by 1915, short for mural painting "a painting executed upon the wall of a building" (1850), from mural (adj.) "pertaining to a wall or walls" (mid-15c.), from Latin muralis "of a wall," from murus "wall" (Old Latin moiros, moerus), from PIE *mei- (3) "to fix; to build fences or fortifications" (source also of Old English mære "boundary, border, landmark;" Old Norse -mæri "boundary, border-land;" Latin munire "to fortify, protect").