|It's a good rule of thumb to|
When people refer to a “rule of thumb” they usually say something like “It’s a good rule of thumb to never shit your pants on a roller coaster that has a lot of loop de loops.” So a “rule of thumb” is basically just a rule. Why is it “of thumb”? What does this have to do with thumbs? The answer is no one really knows. There are a few theories that have to deal with things like tailors using their thumbs to measure cloth, but none of that really make sense in the way the phrase is used.
If you’re going to talk about “rules of thumb” they had better actually be about thumbs. Here are some good actual rules of thumb that everyone should follow:
1. Turning it upward means “good”.
2. Turning it downward means “bad”.
3. Don’t put it in your ass.
Those are the only rules of thumb anyone needs to know. Any other rule you have is just a plain normal rule. Let’s stop including thumbs where they’re not needed (that includes your ass).