The Giant Whistle is a powerskill which summons a hand from the Giant to smash the strongest nearby enemy. This item is exclusive to the Rise of the Giant DLC.
The target is selected depending on different factors such as enemy HP, distance from the player, etc. View this spreadsheet to see how each factor comes into play when blowing the Giant Whistle.
It deals 500 base damage to a single target. Other enemies standing extremely close to the target will be knocked a distance away by the fist. These enemies may take fall damage if possible.
There is a short delay between when the item is used and when the fist attacks its target.
If there are no nearby enemies or if the target did not take damage, the cooldown will be reduced to 1 second.
The game is briefly slowed down as a target is struck.
The Giant Whistle cannot detect invisible enemies by default but will be able to if the player has the "Detect Invisible Enemies" effect on an Amulet.
Occasionally, the Giant's laugh can be heard upon striking an enemy.
Trivia
Because the blueprint for this item is dropped by the Giant, it is technically an item exclusive to the Rise of the Giant DLC, even though it was added to the game in v1.4.
It may be a reference to a dog whistle, which is used to summon pet dogs.
It may be a reference to the Old Bell in Don't Starve, which calls down a giant leg to do damage in the area.