In spite of the fact that these names apply to writing instruments which, nowadays, look something alike, and despite the fact that the three letters of one form the first syllable of the other, the sources of the two words are unrelated.
Pen is derived from the Latin penna, a quill, feather, because, until the invention of steel pens late in the eighteenth century, and long thereafter, until the quality was improved and the price cheapened, sharpened quills had been in use since about the eighth century A.D.
As the point became dulled with use a new point could be made with a sharp knife; hence the term penknife. Before the unknown inventor discovered that a penna, or goose quill, could be sharpened for use, the chief writing implement had been the calamus or sharpened reed, dating back to classic Greek use.
The term pencil, however, was first employed by artists and referred especially to the finely sharpened brushes that they used.
Its source was the Latin peniculus, meaning “a little tail,” because painters’ brushes were first made from hairs from the tails of oxen or horses.
The name was borrowed for the lead pencil when the latter device was invented, about the middle of the sixteenth century A.D., probably because of the softness of the graphite that was first used in them and the resemblance thereby to the soft little brush of the artist.
The medicinal drug, penicillin, was so named because the mold from which it was first obtained resembled numerous tiny brushes.