bedfellow (n.) Look up bedfellow at Dictionary.com
"close friend, roommate, one who shares a bed with another," mid-15c., from bed (n.) + fellow (n.). Also (late 15c) "concubine." Earlier in the "close companion" sense was bed-fere (early 14c.). Old English had simply bedda. Bedsister "husband's concubine" is recorded in Middle English (c. 1300).