pencil

Pencil comes from Anglo-Norman and Old French pincil (compare pincel, ancestor of French pinceau, "paintbrush"), from Latin pēnicillum, a diminutive of pēniculus ("brush"), which was itself a diminutive of pēnis ("tail; penis").

The word traveled through a chain of diminutives — brush, little brush, little tail — before settling into English as the name for a writing instrument.

Pencil is a doublet of penis, both tracing back to the same Latin root; it is not, despite appearances, related to the word pen.