It weighs 0.0s. Using it around dog NPCs (Dog and Doggod NPCs, older dog sisters, and fox brothers) has a 50% chance of making them emote a "?". If the PC's race is either Dog or Doggod, then it will also play a sound.
The whistle is located at 534 (Row 16, Column 6) by the character sprite zero-based numbering system. It is a normal whistle, but tinted black.