pencil See Pen
[ME pensel, pencel, fr. MF pincel, fr. (assumed) VL pinicellus, fr. L penicillus brush, pencil, lit., little tail, dim. of penis tail, penis]
That pen 'enclosure' and pen 'writing implement' are etymologically unrelated may not be surprising, since the meanings are so distinct. The 'enclosure' pen traces its ancestry obscurely back to Old English penn and has no known relatives in other languages. Its homophone, by contrast, goes back to Latin penna 'wing, feather', for the original reference was to a quill pen.
Pen and pencil look close in both form and meaning, but again the resemblance is accidental. When pencil entered English in the early fourteenth centruy (as pinsel, among other spellings), it denoted an artists paintbrush. This was the meaning of its ultimate source in Latin, penicillus. English later also borrowed this Latin word unchanged, referring to a tuft-like structure; the root is more familiar in our word penicillin, produced by the Penicillium mold, which is named for its brush-like shape. Latin penicillus has an anatomical etymology: it is a diminutive of Latin penis, which meant both 'penis' and 'tail'.
ME.......Middle English (A.D. 1100-1500)
MF.......Middle French (A.D. 600-1600)
(Source The Merriam-Webster New Book of Word Histories www.Merriam-Webster.com