I found a more palatable (I guess the pun is intended) interpretation for you. While senna is a plant used for purgative purposes, it's name is considered New Latin from the Arabic word sana (sorry, I can't reproduce the accent line above the second 'a'). In Arabic, sana means 'radiant'. A more common version of this name can be found right here in the Behind the Name database: Sanya.
Senna is also a plant grown for food, as a coffee substitute and as an ornamental plant. It most likely got its name from the attractive yellow to gold flowers that many members of the Legume family have. You can check out the following link for a picture: