Bovine Fecal Matter

Bovine, meaning something has qualities of an ox/cow; bull.
Fecal Matter, obviously is "poop".
When combined, it serves as a lovely way to say "Bullshit."

Also, an abbreviation (BFM) can be used.

Mom: "You're grounded. You should have cleaned your room before you went clubbing."

Me: "Mom, that is bovine fecal matter and you know it."

"Hey, did you hear our football team lost because the ref didn't pay attention? Total BFM."