Expressions & Sayings

I was just thinking, the other day someone said to me, "It's as good as gravy!".
It's got me thinking about saying's and stuff. What if you don't like gravy? Does that mean its shite. I mean don't get me wrong I like my gravy now and again but it gets sickening and I'm sure not everyone in the world likes gravy. If they do I'd eat my hat. Which brings me to another saying, If so n so happens... I'd eat my hat.
How the hell did that come about? I mean the good as gravy one I get, clearly someone that really likes gravy came up with it, but I'd eat my hat? My guess is whoever said this first must have been saying something like... "Aw if he dusny get that promotion i'll eat..." then at this point he see's two things, a big piece of shite and his hat, what one would you go for? Hence they came out with..."Aw if he dusny get that promotion i'll eat my hat!" I wonder if anyone has actually ate their hat out of this expression, if you have please inform me. Cannie believe I actually got out of bed to type this. I felt it was important that you all knew. If I think of any other sayings that I'm no happy about I'll let you know.