Old English elles "in another manner, other, otherwise, besides, different," from Proto-Germanic *aljaz (source also of Gothic aljis "other," Old High German eli-lenti, Old English el-lende, both meaning "in a foreign land;" see also Alsace), an adverbial genitive of the neuter of PIE root *al- "beyond" (source also of Greek allos "other," Latin alius). As a quasi-adjective, synonymous with other, from 1660s; the nuances of usage are often arbitrary.
Productive of a number of handy compounds that somehow never got traction or have been suffered to fall from use: elsehow (1660s) "somehow or other;" elsewards (adv.), 1882, "somewhere else;" Old English elsewhat (pron.) "something else, anything else;" elsewhen (adv.), early 15c., "at another time; elsewhence (c. 1600); elsewho (1540s). Among the survivors are elsewhere, elsewise. Menacing or else, with omitted but implied threat, is implied by 1814:
In Tynedale, Buccleuch seized upon no less than thirty-six English freebooters, and put them to death without mercy. The wrath of Elizabeth waxed uncontrollable. "I marvel," are her own royal expressions, "how the king thinks me so base-minded as to sit down with such dishonourable treatment. Let him know we will be satisfied, or else"--Some of James's ancestors would have bid her
"Choke in thy threat. We can say or as loud."
[Sir Walter Scott, "The Border Antiquities of England and Scotland," 1814]