Well my thought process is that most people would associate the word "mast" to ships - furthermore, to the big long poles that the sails are on. So, I suppose, people would refer to Matty's penis, as a mast, - big long pole.
For the record I half cheated here because I googled poisons and wrote the first one I saw. It turns out it was the right one, but I didn't know it was the right answer.