The Rice University Neologisms Database

Used mostly in text and instant messaging conversations, this exclamation is used in the same context as saying “Yes!” in response to making plans and doing activities. I believe that it is a blend of “Yes!” and “Lit!”, an exclamation denoting that the person using the word thinks that the activity/gathering will be exciting and fun. Therefore, the exclamation “yeet” is both an affirmation and describes how the person thinks the plans/activities will be.