B movie (n.)youdaoicibaDictYouDict[B movie 词源字典]
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.[B movie etymology, B movie origin, 英语词源]