Also, old as the hills. Ancient, as in He must be as old as Adam by now, or That joke is as old as the hills. The first term, alluding to the first human created by God, according to the Bible, was first recorded only in 1867. The variant, referring to geological time (when mountains were created), dates from about 1800.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.