watch one's step - 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.
Exercise caution, as in You'd better watch your step talking to them about a merger. Often put as an admonition, this phrase transfers taking care in walking to other kinds of caution. [First half of 1900s]