I remember reading somewhere or seeing in an interview with Michelle Williams....she said, she found it difficult in rehearsals to say "Jack Nasty" without falling in a fit of laughing, and how she hoped she could say it with a straight face when they were filming.
I think for the times, and because Alma was not a woman who would swear, or say curse words... saying Jack Nasty, was the worst way she could describe him and how she felt about him. It was her way of cursing him.