It's not worth what?
Americans are fortunate to have perhaps the richest language in existence.
In many languages there's one way to articulate something, and that's it. The number of pseudonyms, homonyms, figures of speech, or whatever, are limited. There's one way to describe something, and that's it.
In English, especially American English, there are a host of ways to say or describe something, which gives us the luxury of being extremely specific in our word usage.
Yet we frequently assume that a word has only one meaning, or that there is no flexibility in how a specific word may be employed.
Take, for example the word dam. When we see the word written we know what it means. But when we hear it expressed, we become confused.
Because there's also a homonym, a word that sounds the same, dam^$*%, but has a totally different meaning. This dam^$*% is often used as a curse word, and its usage is eschewed in polite company.
Which brings us to the point of all this. Every so often we hear a phrase used that has a specific meaning, but the user of the phrase is mistaken in his or her assumption that the key word in the phrase has one meaning, when in fact, it has quite another.
The phrase in point is: “It's not worth a tinker's dam.” This is a phrase that has a specific meaning, but its user is often unaware of this. The dam in this case refers to a wad of wet paper used by a tinker to block, or dam, a hole in a metal vessel being repaired by the tinker using molten solder.
The dam in this phrase is often confused with the swear word dam^$*%, so the user incorrectly substitutes the word darn to reconstruct the phrase: “It's not worth a tinker's darn!” Oops!
When one understands the meaning of the two words, the incorrectly sanitized phrase sounds a little stupid. Especially when it's done by a prominent TV personality. That's a BIG Oops.