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.
c. 1300, blenden, "to mix in such a way as to become inextinguishable, mingle, stir up a liquid," in northern writers, from or akin to rare Old English blandan "to mix" (Mercian blondan) or Old Norse blanda "to mix," or a combination of the two; from Proto-Germanic *blandan "to mix," which comes via a notion of "to make cloudy" from an extended Germanic form of the PIE root *bhel- (1) "to shine, flash, burn."
Compare Old Saxon and Old High German blantan, Gothic blandan, Middle High German blenden "to mix;" German Blendling "bastard, mongrel," and, outside Germanic, Lithuanian blandus "troubled, turbid, thick;" Old Church Slavonic blesti "to go astray." Figurative sense of "mingle closely" is from early 14c. Related: Blended; blending.