Definitions by Dame ZNEK

Sick as a dog 

Term for being incredably under the weather. Often used when trying to get out of something such as work or school.

The term came to be because dogs are notoriously constantly ill.
see sarcasm

Fred: I can't come in to work today, I'm sick as a dog.
Jane: Too bad... have a nice trip to the beach!

Sick as a dog by Dame ZNEK January 5, 2009