The term "Manson" is most famously associated with Charles Manson (1934-2017), an American criminal and cult leader. He was the leader of the Manson Family, a cult that was active in California in the late 1960s and early 1970s.
While Charles Manson did not directly commit the murders himself, he orchestrated his followers to commit a series of at least nine murders at four locations in July and August 1969. The most infamous of these were the murders of actress Sharon Tate and four others at her home, as well as the murders of Leno and Rosemary LaBianca the following night.
Therefore, when someone refers to "a Manson," they are almost certainly referring to Charles Manson or something related to the Manson Family and blood line.