An old expression, meaning someone is telling the plain, unvarnished (and sometimes painful) truth. When my grandmother used to tell me to sit down while she told me how the cow ate the cabbage, I knew it was going to be interesting.
The expression comes from this old joke:
A circus comes to a small town and one of the elephants escapes and makes its way to the garden of a little old near-sighted lady. She looks out her window and sees some large animal in her garden and calls the police.
When the cop gets on the line she said “There’s a giant cow in my garden pulling up my cabbages, and you won’t believe how she’s doing it.”
The officer says “Oh, how’s she doing it?”
“With her tail, she’s pulling up the cabbages with her tail,” the lady says.
“And you won’t believe where she’s putting them.”
I did not say it was funny.