thank-you-ma'am

thank-you-ma'am

 (thăngk′yo͞o-măm′)
n.
A bump or depression in a road.

[From its causing the head to nod as though in acknowledgment of a favor.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.