I probably would've had to stay after school and remove all the gum from underneath the desks. So, if your last name ends with Stanley, Monroe, Flatt or Scruggs please bear with me here. I also want to shout a warning out to anyone that has a definition of bluegrass in their bean already. My definition is mine, meaning that some will certainly find fault with it, whereas others will think I didn't go far enough. To combat all of that I will skirt the issue at hand whenever possible.
Bluegrass music is really just modernized oldtime music.
In fact, most people trace bluegrass music back to Bill Monroe who took the oldtime stringband music that he heard growing up as a kid in Rosine, Kentucky and souped it up. He took if from being a type of music that was played at dances, to one that was meant to be performed for an audience. At the beginning the same songs and tunes were used as in oldtime, but Bill sped them up, added some blues, and some jazz along with improvisation, fancy sangin' and soloing. In bluegrass no longer were all of the lead chores delegated to the fiddle and banjo. Now, anyone could take the lead.