"How now brown cow" is a phrase used in elocution teaching to demonstrate rounded vowel sounds. Each "ow" sound in the phrase represents an individual diphthong. The phrase does not have an explicit meaning per se but can be used as a light-hearted greeting. Although the exact origins of the phrase are unclear, it can be dated to at least 1942 in the United States. In February of that year the Maryland newspaper The Capital mentioned the phrase when discussing a famous thespian's voice:
Laird Cregar, now contributing his booming voice to 'Ten Gentlemen from West Point': explains how he got it. When he first tried out for the Pasadena Community Playhouse his voice wouldn't carry past the front rows. Coach Belle Kennedy had him declaim 'How, Now, Brown Cow? and The Rain in Spain Still Stains' - over and over.
Hmph. Who'da thunk?
I took this picture of these curious cows in Zillah, Washington at my nephew's house. He has steers and miniature horses. Again, the lighting was just beautiful - this photo was taken around 9:30 in the morning.