Stories We Like: A Guide to the Comma
by 1939, usually said to be so called from being the second, or supporting, film in a double feature. Some film industry sources say it was so called for being the second of the two films major studios generally made in a year, and the one cast with less headline talent and released with less promotion. And early usage varies with grade-B movie, suggesting a perceived association with quality.
A usually low-budget movie intended for the broad middle ground of taste and meant to be primarily entertaining and narrative rather than serious, artistic, etc
[1930s+; fr their being the second movie in a double feature]