What do the boots represent in the poem "Primer Lesson" by Carl Sandburg?

This poem is a warning against speaking harsh, “proud words” – Sandburg writes in lines 2 and 3 of the four-line poem:

When you let proud words go, it is not easy to call them back. 
They wear long boots, hard boots; they walk off proud; they can't hear you calling—

The boots here represent the durability of such proud words – the “long…hard boots” call to mind thick-soled leather work boots with steel toes; boots that you can walk for miles in.  In like manner, proud words can walk for miles; that is, they are not easily forgotten, and their effects are lasting.  What’s more, “they can’t hear you calling.”  You cannot take such words back.  Once they are released, they will not dissipate, and their meaning will stick in the minds of those who heard them for a very long time.

Pride, and words uttered in pride, can do lasting damage to relationships and to one’s reputation, and once they are said they cannot be taken back.  They endure, and they travel far, just as heavy boots endure and travel far.  Word can get around, and can spread beyond your jurisdiction.  Thus rash words can cause lasting, wide-spread damage.