put on one's thinking cap
Think or reflect seriously, as in A new slogan? I'll have to put on my thinking cap for that. This term originated in the late 1800s and replaced considering cap, which dates from the early 1600s.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.