between (prep.) Look up between at Dictionary.com
Old English betweonum "between, among, by turns," Mercian betwinum, from bi- "by" (see be-) + tweonum dative plural of *tweon "two each" (compare Gothic tweih-nai "two each"). Between a rock and a hard place is from 1940s, originally cowboy slang. Between-whiles is from 1670s.