The deadly nightshade you slipped me wore off, Sally.
Nightshape is a family of poisonous plants. (Tomatoes are nightshades; the leaves are poisonous.) Sally must have made a potion of nightshade and put it in his orange juice or something. It would probably have a sedative effect or something. (Kids, DON'T try this at home.) It wore off, though. (To wear off= to lose its effect. He wasn't drugged any longer.)
I have swept the very bravest off their feet"
"To sweep someone off their feet" usually means to make someone fall madly in love with you. In this movie, though, the creepy skeleton guy says it. He seems to mean, "I am so good at being scary, that I have made even the bravest people collapse in fear."