Totally secure, as in If you buy Treasury bonds, your money will be safe as houses but you won't get a large return. In today's security-conscious climate, where alarm systems to deter housebreaks have become increasingly common, this simile may seem puzzling. Presumably it uses house in the sense of “a shelter from the elements.” [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.