"one who hires a room, a lodger," 1871, agent noun from room (v.).
Entries linking to roomer
room v.
"to occupy a room or rooms" (especially with another) as a lodger," by 1825 (implied in roomed), from room (n.). Related: Rooming. Rooming-house, "house which lets furnished apartments," is by 1889, according to OED "chiefly U.S." In Old English (rumian) and Middle English the verb meant "become clear of obstacles; make clear of, evict."