lead with one's chin
Take a risk, behave without caution. For example, Gordon always says exactly what he thinks; he never minds leading with his chin. This term alludes to a boxer leaving his chin, a vulnerable point, unprotected. [Mid-1900s]
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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.