What the hell does liberty mean anyhow? It's just a word like house or table or any other word. Only it's a special kind of word. A guy says house and he can point to a house to prove it. But a guy says come on let's fight for liberty and he can't show you liberty. He can't prove the thing he's talking about so how in the hell can he be telling you to fight for it?
This passage, from Chapter x, indicates Joe's extreme skepticism about abstract language and the intent of those who use it for persuasion. From Joe's point of view, the farther removed words are from the things they describe, the more room there is to for the meaning of the words to shift to accommodate the desires of whoever is using them. Joe's point is that liberty can mean one thing to one person and another, very different thing to another person, therefore making the end value of the word nothing. If the word cannot be trusted to mean something—if liberty cannot be truly promised—then it is not remotely worth risking one's life for.