If something improbable should occur. For example, In the unlikely event that I'm held up, please cover for me, or In the unlikely event that we should have snow in May, we're still well equipped to cope with it. Also see under in case, def. 2.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.