Rectal is a doctor's word. It means “about the rectum” — the last part of the bowel, just before the opening in your bottom. You meet it on medicine boxes and in hospitals more than in ordinary talk. A rectal thermometer is one you put in the bottom; a rectal exam is when a doctor checks that part of the body with a gloved finger; rectal medicine is meant to go in that way, not in the mouth. The word itself is not rude. It is cold and clinical. At home people usually dodge it and say “bottom” or “from behind.” You would not call a friend rectal; you only use it for the body part or the medical route.