The incorporation of Latin root words into English is still going on. Latin was the language of the church and the educated all through the Middle Ages. The language acquired words whose meaning has been lost and are guessed at today. English picked up Latin from the early scholars, from the Roman invaders, from the Norman invaders, from contact with European languages derived from Latin, and, not least, the new Latin added by scholars. I have added a few myself, as has anyone involved in describing the diversity of life, and working in research in most branches of science. Some is Latin borrowed from Greek, so it may be said that these languages are still alive and contributing.

Penicillium is a good example of a modern botanical Latin word, from Latin penicillum, a painter's pencil or brush. A penicil is a small tuft of hairs, scales, etc. The relationship to our word pencil is obvious. A penicillus also is a small tent-like covering for wounds. Penicellate is an adjective for having a small tuft. Penicillin appeared in 1929 in a British medical research journal, a deliberately coined name (by Fleming) for antibiotics derived from the mold Penicillium.

It is easy to invent relationships ("akin or derived from") among old words such as penicel and penis. Pinnate, peninsula as well as penicel are related to penis. Trying to trace Indo-European roots is fraught with speculation. Pen for an enclosure and pen for writing?