1. sick as a dog - Adjective
2. sick as a dog - Phrase
sick as a dog (not comparable)
(simile) Very ill.
I caught that new strain of flu and for three days I was as sick as a dog.
I'm down here in Mexico, sick as a dog. My head is poundin' in this border town fog. Down to my last dime and comin' apart at the seams. I've messed up in Mexico, livin' on refried dreams. Tim McGraw
When it came time to record the song, Batey was apparently “sick as a dog.” Source: Internet
When he won at his third attempt, for Christopher Nolan's in 2018, he spent the next fortnight in bed, sick as a dog. Source: Internet