An Etymological Dictionary of Astronomy and Astrophysics

1a) A slender tube of wood, metal, plastic, etc., containing a core or strip of graphite, a solid coloring material, or the like, used for writing or drawing.

1b) Anything shaped or used like a pencil.

2. A narrow set of lines, light rays, or the like, diverging from or converging to a point (Dictionary.com). → pencil of light.

Etymology (EN): M.E. pencel, from M.Fr. pincel, from L. penicillus “painter’s brush or pencil,” diminutive of peniculus “little tail,” diminutive of penis “tail;” → light.

Etymology (PE): Medâd “pencil,” of unknown origin.
Kelk “quill, pen,” originally “hollow reed.”
Bâriké, from bârik, → narrow, + nuance suffix -é; nur, → light.