1. having a tendency to laugh; disposed to laugh
2. arousing or provoking laughter; funny
3. associated with, relating to, or used in laughter (risible muscles, for example)
Etymology: mid 1500s, Latin, from ridere, to laugh.
As far as I'm aware, the term "risible muscles" isn't a medical term for any particular muscle group; it's just an informal term for the muscles used in laughing.