An exclamation of enthusiastic agreement which can be used in both a genuine or sarcastic tone. In addition can used to add emphasis to a statement, much like the word fuck
Boy Howdy was also the name of an American country music band from the 1990’s. The term has not direct link or reference to them however.
As Agreement (genuine):
Joe: Mrs. Robinson sure is one MILF
Tom: Boy howdy!
As Agreement (sarcastic):
Johnny: Don't you just love pointless office meetings where the company tries to rah rah rah you into caring about sales?
Roberto: Boy howdy do I ever!
Norman: Boy howdy am I hungry, let's consume tacos!