Bet you knew the rest of the phrase…
But maybe not the meaning.
Wikipedia (who knows EVERYTHING, by the way),defines the phrase as-
“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. Its use in teaching elocution can be dated back to at least 1926.
Now you know…..
So, go put on your diphthong and start learning.