interpenetrate (v.)

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.

1520s, "to pierce into or through," from Latin penetratus, past participle of penetrare "to put or get into, enter into; cause to go into." This is related to penitus "within, inmost, interior," penetralis "penetrating; innermost;" penus "innermost part of a temple, store of food," penarius "used for storing food;" Penates "household gods."

All are from penus/penoris "food, provisions," from Proto-Italic *penos, from PIE *penos "food" (source also of Lithuanian penėti "to feed"). De Vann writes that "The semantic appurtenance to 'feed' is explained by Stüber as 'what one feeds with' ('food') > 'the place one feeds at' > 'interior, home'."

The figurative senses of "enter and affect deeply, influence, impress" and "gain intellectual or spiritual access" are from 1580s. Related: Penetrated; penetrating.