No observers or authorities are present; one can proceed safely. For example, Let's make sure the coast is clear before we set up this surprise party. This expression may have originated among pirates and smugglers who were referring to the absence of coast guards, or with regard to a coastal military invasion, but no citations bear out these theories. By the late 1500s the term was used purely figuratively.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.