Terry Bradshaw -- Reggie Bush "Chasing Bucket of Chicken" Adlib ... REALLY Bad Inside Joke

Terry Bradshaw made the biggest fumble in the NFL on Sunday when he compared Reggie Bush TD run to "chasing a bucket of chicken" -- but the jaw-dropping comment was intended to be an inside joke ... TMZ has learned.

Hall of Famer Bradshaw was describing Reggie's play during yesterday's halftime highlights show on FOX NFL Sunday -- when he spewed, "Look at this Jimmy, like he was chasing that bucket of chicken that the wind was blowing the other day."

Sources who were inside the FOX studio yesterday ... tell TMZ Terry was not trying to insult African-American Reggie -- but instead was taking a shot at fellow FOX analyst Jimmy Johnson.