General:
* Scribble: To write quickly and carelessly, often illegibly.
* Scrawl: To write hurriedly and untidily.
* Dash off: To write something quickly and without much thought.
* Jot down: To write something quickly and briefly.
* Note down: To write something down quickly, usually for future reference.
More specific:
* Stenograph: To write using shorthand.
* Speed write: To write very quickly, often using a specialized system.
* Freehand: To write without using any guides or tools, often quickly.
Figurative:
* Pen a quick note: To write a short and simple message.
* Knock out a draft: To write something quickly and roughly.
* Spit out words: To write something quickly and without much thought.
The best word to use will depend on the specific context and the intended meaning.