soap opera: [20] The original soap operas were a radio phenomenon, in 1930s America. Serial dramas dealing with humdrum-butoccasionally- melodramatic domestic life were as common then as they are on television now, and several of those on the commercial US networks were sponsored by soap manufacturers. A writer on the Christian Century in 1938 said ‘These fifteen-minute tragedies…I call the “soap tragedies”…because it is by the grace of soap I am allowed to shed tears for these characters who suffer so much from life’.
The soap connection soon linked up with horse opera, a mildly derisive term for a Western movie that had been around since the 1920s, to produce soap opera (a later coinage on the same model was space opera). The abbreviated version soap is recorded as early as 1943. [soap opera etymology, soap opera origin, 英语词源]
"melodramatic radio serial" (later extended to television), 1939; so-called because sponsors often were soap manufacturers, from earlier horse opera "a Western" (1927). Shortened form soap for this first attested 1943.