Signing: Messy looks like you are upturning a fish-tank, making a big mess. Start with your hands open, with one hand up and one hand down. Then with a round motion, make you top hand go down, and your bottom hand end up at the top.
Usage: We often use messy to describe a situation before we tidy up. We might describe the playroom as messy, with a smile before we start tidying. Or we might use messy when we see baby with food all over their face, before we start cleaning them up.
Messy is just an observation (and not a criticism) that marks the transition to clean-up time.
Flash Card: Click the link to view the Messy Baby Sign Language Flash Card. The flash cards are printable and available in both U.S. Letter and A4 sizes.