I rattle off phrases all of the time without knowing where they really come from. Lots of times you can decipher their origins pretty easily - muffin top, for example, doesn't take a whole lot of brain power to figure out. But I could have never guessed the origins of some of these phrases, which I now know thanks to Barbara Ann Kipfer's Phraseology book. I've written about this book before - it's endlessly fascinating.
If there's a particular phrase that has you stymied, let me know in the comments - I'll try to find its origins (whether it's in Phraseology or not) and feature it in a future post. I can't promise anything, but I'll try!