I'll pencil you in

This is what a person says when he is making a tentative appointment.

"I'll Pencil" (because the appointment can be erased) "you in" (to my schedule book).

When somebody wants to schedule something with you, but you don't want to use a pen to finalize it in your calendar because that person is a loser. If something better comes along -- you can erase it.

tenatively allocate time to meet, see, or conversate

A nice way to say "fuck off...im not interested"

To schedule someone for a meeting.
To enter an appointment into an appointment book with pencil.

Informal idiom meaning to make an appointment for someone; to "pencil them in" to the schedule.

A phrase meaning that the speaker intends to add the listener to a list, or make an appointment for the listener to attend a meeting at a later time.

(Pencil, a noun, is being used as a verb for the action associated with it.)