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8.
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.)
Fred: I need help with my homework.
Daphne: I'm not free until 6.30. I'll pencil you in for then if that's OK.
Fred: It's a date.
by phantom oddity May 20, 2004
 
9.
It literally means that someone will give you the details of something, but is usually just someone trying to give you the runaround.
"What happened with you and your girlfriend?"
"I'll pencil you in later."
by Jessica May 20, 2004
 
10.
To sexually assult/stimulate someone with a pencil, usually rubber end first for safety.
Oi, come here you silly slag... I'll pencil you in!
by Horny B April 27, 2008