The name penicillin comes from Latin penicillus, brush, and is related to the word penis
Penicillin is so called because of its appearance: under the microscope, a colony of Penicillium looks like a brush. The name of the genus, Penicillus, comes from the Latin penicillus, "paintbrush". Oddly enough, penicillin is a kinsman of the word penis, from the Latin penis, "tail". Penicillus, accordingly, a "little tail", alludes to the tufts of hair that have long been used to make brushes.