That has to be something I heard..
Oh it was an old one..
"This idiom was popularized after the release of the White House tapes in connection with the Watergate Scandal of the early 70's, which contained H.R. Haldeman's conversation with Presidential Counsel John Dean. Haldeman tried to dissuade Dean from testifying to the Senate, saying “Once the toothpaste is out of the tube, it’s going to be very hard to get it back in.”