1550s, "influx of water; place where water flows in," from in (prep.) + set (n.2). The sense "that which is set in" ("extra pages of a book, etc.," 1871; "small map in the border of a larger one," 1872) probably is a separate formation. In Old English insetan (Old Northumbrian insetta) meant "an institution," literally "a setting in," and perhaps a loan-translation of the source of institution. Similar formation in German einsetzen "to use, employ; institute, begin; install."