doodle

v.

To draw something aimlessly or absent-mindedly, usually while doing something else such as having a telephone conversation or attending a meeting.

n.

A drawing, sketch, or scribble made aimlessly or absentmindedly.

graffiti

n. Plural of graffito.

An inscription or drawing made on some public surface (as a rock or wall).

Graffiti, which also serves as the plural of graffito, is commonly used as a singular mass noun. It is also used as a transitive verb.