What does “iron-ass” mean?

In New York Times’ (November 7) article under the title, “Poppy Bush finally gives junior a spanking,” Maureen Dowd introduced the following statement of Jon Meacham’s new biography, “Destiny and Power: The American Odyssey of George Herbert Walker Bush.

”While G.W. Bush used to say that what he liked about Dick Cheney and Donald Rumsfeld was their brass appendages, Poppy offered a dimmer anatomical appraisal, calling each an “iron-ass.”

I’ve never heard the word, “iron-ass.” I don’t find this word in neither Oxford nor Cambridge online English dictionary.

Google Ngram shows that the word emerged around 1915 and continues intermittent usage at a nominal level of 0.000000045 in 2000 in contrast to “iron butt,” of which incidence rate is a digit higher (0.000000376).

Incidentally, Urban dictionary defines “iron butt” as a nickname for a long-distance motorcyclist. It often specifically applies to someone who has motorcycled 1000 miles in a 24-hour period.

What does “iron-ass” mean? Does it mean wrongheaded or ironhanded? Is this popular word, though Google Ngram indicates otherwise?

Solution 1:

The meaning of the word is on "iron" rather than "ass". There are a lot of words that have "X + ass" such as dumb-ass, hard-ass, bad-ass and the list goes on and on. In these words, the X has a meaning and "ass" means just "man", "person" or "friend", etc.

Iron is broadly ued to mean:

Used figuratively as a symbol or type of firmness, strength, or resistance: ‘her father had a will of iron’

[Oxford Online Dictionary]

"Iron-ass" means someone with a will of iron".

Solution 2:

This is an ad-hoc escalation of the term "hard-ass," to convey the meaning that they're "extra" hard-ass.