As I understand the term, it's a buck with an inferior rack, whose genes you don't want passed on. Example, on the lease I hunt, the guys have determined that a 2 1/2 - 3 1/2 year old "slick six" (6-point with no brow tines) is a "cull buck".

I'm not so sure, as I saw a nice slick six last year that I believe had brow tines this year...