Originally used as the
tagline for a very cheap beef-flavoured noodle product which was safe for
vegetarians. The phrase is now used to describe that there is no meat in a product.
The sister phrase "Ain't no chicken" originates from the tagline of the chicken flavoured noodles. And is used in the same way but only when the product is chicken flavoured.
Eg.1
Man: Would you like some of my beef-flavoured
noodles?
Vegi: Is it safe for vegitarians?
Man:
No Bull!
Eg.2
Man: Would you like some chicken flavoured
crisps?
Vegi: Is it safe for vegitarians?
Man: Ain't no chicken!