Too big for (one's) britches Overconfident; cocky.(idiom)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "britches" in a sentence
"Harry Reid reminds me of a constipated “old-maid” in britches."
"I suppose we got our old fashioned expression "britches" from that."
"Unknown to the stranger the condition of his "britches" had probably given him his credit rating with Old Coonrod, for he held that patches upon the front of trousers, if the seat were whole, were decorations of honor, showing the man had torn them doing something, going forward."