I’m a big Heinlein fan. I thought it was an odd saying when I read ‘Stranger’ so I researched it some years ago. I was surprised to see it in your post...but recognized it instantly.
I love Heinlein too, one of the masters of science fiction.
I’ve found a good few of the old authors at the Baen Books website.
Ah the good old days when we were young and the universe was ours for the taking.