late 14c., propinquite, "nearness in relation, kinship," later also "nearness in place, physical nearness" (early 15c.), from Old French propinquite (13c.) and directly from Latin propinquitatem (nominative propinquitas) "nearness, vicinity; relationship, affinity," from propinquus "near, neighboring," from prope "near," with loss of second -r- by dissimilation, from PIE *propro "on and on, ever further" (source also of Sanskrit pra-pra "on and on," Greek pro-pro "before, on and on"), from root *per- (1) "forward," hence "in front of, toward, near." The signification of the suffix -inquus is unclear.
Nothing propinks like propinquity [Ian Fleming, chapter heading in "Diamonds are Forever," 1956; the phrase was popularized 1960s by U.S. diplomat George Ball]