In the days when mounted knights participated in jousting matches…

Dim memories of researching the word career in the OED lead me to believe this etymology is all folked up. It was, as I remember it, originally a noun referring to a portion of a horse race track and it got itself verbed fairtly early in its career (this definition of career is a derived definition from the race track sense). Y'all'll have to wait for tonight for confirmation of this unless tsuwm or some other person with ready access to the OED can come along and back me up.