I got Words we Use, by Diarmaid O Muirithe out of the library which is smashing book for all putative etymologists.
In it he says that not long ago he heard a Donegal chef shout from his kitchen that the lettuce had gone silly.
Apparently in this sense silly is a totally correct word for limp and flabby.
I still wish I had heard it.
The notion of lettuce being silly in the cold room really appeals to me.