POE: Phrases of Excitement.
An utterance of great surprise, an exclamation of joy, awe, or fright.
The first known use of Boy Howdy was in 1911. A Southernism, usually attributed to Texas and evidently popular in that state, it’s a simple combination of the exclamation “Boy!” (indicating surprise) and our friend “Howdy,” together used to mean “Wow!” or to indicate strong agreement with a statement or question (“Was your mom mad at you?” “Boy howdy! I’m grounded for a month.”). The phrase seems to have been popularized in the years after World War I, when returning soldiers who had heard it from Texans in the service brought it back to civilian life.