DAVINA: “Trammel” comes from the Latin “trimacula”, which means three meshes. It referred to a net used for catching fish. 
RALPH: In the Elizabethan era, besides referring to a fishing net, trammel was used figuratively for other constraining devices, especially a kind of shackle placed on a horse’s hind legs to keep it from kicking, or “starting” as Shakespeare might have said. 
DAVINA: This figurative sense of “trammel”, referring to a restraining device for an animal, goes all the way back to ancient Roman times, when it referred to a long stick tied across a dog’s neck to prevent it from being able to enter the rows of a vineyard and damage the grapes. 
RALPH: Here, Shakespeare uses the term as a verb, to mean “catch”, as in a net. Macbeth is wishing that the act of assassinating Duncan could catch or constrain the consequences, so that he wouldn’t have to worry about them.