Not So Tough As Nails

I've been thinking alot lately about the phrase "Tough as nails".  It makes no sense, in this modern age, today.  Perhaps if I was Amish or a Mennonite and wooden dowels were all I had, "tough as nails" might carry more purport.  Perhaps if I lived 1000s of years ago before bolted joints, hot riveting, brazing, epoxy joining, press-fitting, or arc/mig/tig/friction/and EB welding existed on Earth, "tough as nails" might carry a connotation of "as tough as a bear's nails" or "as tough as an eagle's talon".  But humans today are very disconnected from bears, eagles, or other wild animals (save for their sport teams' mascots).  So just what is "tough as nails" supposed to mean?

Does it mean that it bends easily when not hit precisely square with a hammer, and then it quite difficult to extract without causing damage to the wood in which it was pounded?

Does it mean 'that which should never be used in shear' as the nails under Dr Desert Flower's friend Brook's hot tub were used in shoddy construction where the entire weight of hot tub, water, and drunken revelers was supported solely by three 16D nails into each corner's platform support upright?  (which failed in shear, dumping the hot tub, water, and inhabitants one night in 1997)

Does it mean 'that which if pounded into the wood too close to the edge will indeed cause the wood to Break Out and split'? (Yes, my father and god father are excellent wood workers, a skill that was not passed down to me)


Or is there no construction component whatsoever and instead it applies to mammalian and avian nails?  Bears, eagles, badgers, seals, walruses, hawks, humans, etc?  Most of the quadrupeds and raptors have somewhat formidable nails, composed mostly of keratin, but human nails are fragile, flimsy, transient, ornamental vestiges that break even easier than construction nails.  And the phrase "as tough as nails" has no species qualifier to it.  Humans being as myopic as they are, I think that the phrase must have originally been used to describe human nails.

Un-cut baby nails can claw at their parents a great deal.  And humans who have been pre-maturely buried have tried to claw their way through their coffins, when wooden coffin burials were the custom in cultures who did not cremate their dead.  But I just do not see any resemblance to "toughness" or "durability" in human nails that the phrase "tough as nails" incorrectly implies.

If anyone has any insights as to the rightful origin of "Tough as nails" I would be very interested to hear it.   My "comments box" is always open. - JJP