People or persons who have no meaning in this world, they have a boring personality, and they are rather dull. People who are outgoing are usually not fill-ins.

A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.

to attack somebody physically and successfully

Refers to an act of grafitti art. To use two or more spraycan colors to fill-in an outline of text on public property.

Slang for 'feeling'. Not 'feeling' in the context of feelin' on yo booty. Feeling as in "I feel like doing it today" or "I don't feel like doing that".