A large sliding or hinged door of a barn, or figuratively, something very large and obvious to hit or target.
Compound word from barn (Old English bere-ærn) plus door (Old English duru), describing the large entrance doors of farm buildings.
The phrase 'couldn't hit the broad side of a barn door' comes from the fact that a barn door is so huge that missing it seems impossible—yet somehow people still manage to miss!
Complete word intelligence in one call. Free tier — 50 lookups/day.