To scrawl is to write in a quick, barely readable scribble. When you're signing a document, you might scrawl your name across the bottom.
Doctors are well-known for the way they scrawl prescriptions on a pad, and you can refer to that chicken scratch handwriting itself as a scrawl. It's not easy to read someone's scrawl, which is careless and rushed. The origin of scrawl is (fittingly) unclear, although one guess connects it to the Middle English scrawlen, "spread out the limbs" or "sprawl."