c. 1200, from till (prep.). The first element is un- "as far as, up to" (also in unto), from Old Norse *und "as far as, up to," from Proto-Germanic *und- (source also of Old English oð "up to, as far as," Old Frisian, Old Saxon, Gothic und), from PIE *nti-, from root *ant- "front, forehead," with derivatives meaning "in front of, before."
The two syllables have the same meaning. Originally also used of persons and places. As a conjunction from c. 1300. Similar formation in Swedish intill, Danish indtil (northern English and Scottish formerly also had intill/intil "into, in"). The Modern German equivalent, bis (Old High German biaz), is a similar compound, of Old High German bi "by, at, to" and zu "to."