watch it - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, watch out. Be careful, as in Watch it as you go down that ladder, or Watch out, there are a lot of cars on this road. The first term dates from the early 1900s, the variant from the mid-1800s. Also see look out.