Used to refer to a sailboat which has lost all speed because it is headed into (or close enough to) the wind.
A sailboat with its bow pointed directly into the wind, preventing the sails from filling properly so that the boat can move. It can be very difficult to get a boat that is in irons back under sail. An old square rigger could take hours to get underway again.
When a boat's bow is in the wind's eye and, having lost all headway, will not go off on either tack