Perfectly okay to say this as long as you are talking about your own mother! Maybe this is okay to say this about someone else’s Mother if she is actually wearing army boots at the time you say it. But it is never okay to say this about someone else’s mother if they are not or have never worn them at all!
Recently a black person lost their job for using the N-word. Once again perfectly okay if he was referring to his own Mother, it might even be okay for them to say this about another black persons Mother! But as a white person I wouldn’t like him calling my Mother the N-word or even more appropriately calling her the C-word a Cracker!
In a perfect world nobody’s Mother should be wearing army boots! Unless of course they are in the Army. But of course in a perfect world there would be no armies! And nobody should be using the N-word or C-word either!
And that is all I have to say about that!