One of the 20th century's most important drugs is named after a paintbrush, and its closest relative in English is the pencil.
On 3 September 1928, Alexander Fleming returned to his laboratory at St Mary's Hospital in London. One of his culture plates had been contaminated by a blue-green mould, and bacteria near it had been killed.
Fleming worked out a way to grow the mould in large cultures and collect its "mould juice." On 7 March 1929 he gave it a proper name, to avoid repeating the "rather cumbersome phrase" mould broth filtrate. In his Nobel lecture he explained that the new word showed the substance came from a plant of the genus Penicillium. He compared it to digitalin, named long before for a substance from the plant Digitalis.
The genus name is the interesting part. Penicillium comes from Latin penicillus, "paintbrush," because the mould's cells look like tiny brushes. Penicillus was itself a diminutive of peniculus, "brush," from a Latin word meaning "tail." A Roman paintbrush was, in effect, a little tail.
Fleming published his findings in the British Journal of Experimental Pathology in June 1929. Even the mould's species name proved slippery. Fleming's paper called it Penicillium rubrum. In 1931 Charles Thom identified it as P. notatum, the name Etymonline still gives. Genome sequencing in 2011 placed it in a third species, P. rubens.
The word waited a decade for the drug to catch up. In 1939 a team at Oxford led by Howard Florey, with Ernst Chain, took up Fleming's paper and mould sample. Their work showed the drug's curative power. Fleming, Florey and Chain shared the 1945 Nobel Prize.
The paintbrush root runs elsewhere in English too. The Latin penicillus became Middle English pencel, an artist's fine brush of camel hair. By the mid-16th century the pencil was also a writing tool, but its name still goes back to a little brush.
Later antibiotics borrowed the ending. Amoxicillin, recorded from 1971, is a contraction of a chemical name that ends in penicillin.