Bellow comes from Middle English, and means "to roar like a bull." A cow's deep moo is called a low, so bellow just adds a big angry bull to the standard moo.

What does my bellowed mean?

verb. (intr) to make a loud deep raucous cry like that of a bull; roar. to shout (something) unrestrainedly, as in anger or pain; bawl.

What is definition of bellowed?

1 : to make the loud deep hollow sound characteristic of a bull. 2 : to shout in a deep voice. transitive verb. : bawl bellows the orders.