In Carl Sandburg’s short poem "Primer Lesson," the implied metaphor is the comparison between what Sandburg refers to as “proud words” and a long, "hard" pair of boots. When Sandburg talks about words that are proud, he implies that he means harsh words and words spoken out of ego instead of empathy. He prompts the reader to picture such words as hard boots because the meaning and effect of such words tend to be harsh and strong.

Through this comparison, Sandburg’s speaker warns the reader to be careful about how to use proud statements. Like a large pair of boots, the words will “walk off proud,” and one is not able to call such words back. Now of course, words won’t actually grow feet with work boots on them and walk away from the person who said them. But what Sandburg means here is that proud language tends to have a significant, lasting impact.

Consider how words spoken out of pride tend to be remembered. For example, in the heat of an argument with a loved one, a person might utter something hurtful out of pride. These words could hurt that person’s relationship because proud words tend to impact emotional bonds. They are not the kind of words that one could pass off as sarcastic or meaningless after they are said. Just like a large pair of boots that noisily clomp away, the words cannot be easily ignored.