Pencil in

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.

a stick of cosmetic coloring material for use on the eyebrows, eyelids, etc.

anything shaped or used like a pencil, as a stick of medicated material

a styptic pencil.

a narrow set of lines, light rays, or the like, diverging from or converging to a point

a pencil of sunlight.

a slender, pointed piece of a substance used for marking.

style or skill in drawing or delineation

He favored the late products of the artist's pencil.

Mathematics . the collection of lines, planes, or surfaces passing through a given point or set of points and satisfying a given equation or condition.

Archaic . an artist's paintbrush, esp. for fine work.

to write, draw, mark, or color with, or as if with, a pencil.

to use a pencil on. ?

pencil in, to schedule or list tentatively, as or as if by writing down in pencil rather than in ink

I'll pencil you in for ten o'clock.