i have always described messy writing a “scrawl”; in the back of my head have always thought it related to chickens scratching at the floor, looking for worms and seeds.
though I have never heard of “chicken scratch”!
here in Yorkshire, we have a words like,
“aft” = afternoon
“muckle” = large
and quite a few more;
http://en.wikipedia.org/wiki/Yorkshireisms
now; if you go back 50 years, each dale/valley (only 200 or so people) in the yorkshire dales had its own language ; a bit like china is today!