Between
Old English betwēonum, from be ‘by’ + a Germanic word related to two.
wiktionary
From Middle English betwene, from Old English betwēonan, betwēonum(“between, among, amid, in the midst, meanwhile”, dative plural, literally “by the two, near both”), from Proto-Germanic *bi-(“be-”) + *twīhnaz(“two each”), corresponding to be- + twain. Cognate with Scots between(“between”), Scots atween(“between”), Gothic 𐍄𐍅𐌴𐌹𐌷𐌽𐌰𐌹( tweihnai, “two each”), Old English betweohs(“between”), Old English twinn(“double, twofold”). More at betwixt, twin.
etymonline
between (prep., adv.)
Old English betweonum, Mercian betwinum, "in the space which separates, midway, in the midst, among; by turns," from bi- "by" (see by) + tweonum dative plural of *tweon "two each" (compare Gothic tweih-nai "two each;" from PIE root *dwo- "two").
Between is literally applicable only to two objects; but it may be and commonly is used of more than two where they are spoken of distributively, or so that they can be thought of as divided into two parts or categories, or with reference to the action or being of each individually as compared with that of any other or all the others. When more than two objects are spoken of collectively or in divisibly, among is the proper word. [Century Dictionary]
In all senses, between has been from its earliest appearance, extended to more than two. [OED]
Between a rock and a hard place "caught in a dilemma, in a difficult situation" is from 1940s, originally cowboy slang (earlier was between the beetle (hammer) and the block, late 19c.). Between-whiles "at intervals" is from 1670s.
