At the beginning of the My Fair Lady movie, there is a monologue of prof. Higgins like this:

```
Hear a Yorkshireman, or worse
Hear a Cornishman converse
I'd rather hear a choir singing flat 
Chickens cackling in a barn 
Just like this one 
Garn!

```

My question is: what is the meaning of this "Garn"?