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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.