There are many different emotes that you can acquire as you play through Lost Ark. You can use these emotes for a variety of reasons including building rapport with NPCs and progressing through different quests.
While you can acquire most of them through the story, you can't acquire them all. One emote that you need to go out of your way to look for is the Whistle Emote. First, it's important to note that you need to have reached the endgame and have an Item Level of 800. Once you've gotten that taken care of, you can head to Azure Wind Island which is located south of Rohendel and north of Yorn.
On the island, you will need to begin the main questline titled Dying Horse up to the second quest titled The Mane of the Azure Wind. At that point, you will have access to a task from Una's Task list titled Ruffians in the Ground. This quest requires you to destroy a total of x25 Root Borers that are randomly located around the island. You need to complete this task until you reach reputation level 3.
Once you do, you will receive the Whistle Emote as a reward. It will be in your inventory at first so you will need to right-click on it to bind it to your character for use.