means An antibiotic drug derived from Penicillium mold, used to kill or stop the growth of bacteria and treat infections that were once routinely fatal.

from From the mold genus Penicillium, named by Latinists for its look under the microscope: penicillium is Latin for 'little brush' or 'painter's brush' (a diminutive of penicillus, itself a tiny version of peniculus, 'brush' — and yes, distantly tangled with the Latin root that also gives us 'pencil'). The spores fan out like fine bristles, so the mold wore the name long before its secret was found. Alexander Fleming coined 'penicillin' for the bacteria-killing substance the mold quietly produced.