ik what you did last summer

It looks like you are asking for the meaning or usage of the phrase "ik what you did last summer." "Ik" is commonly used as an abbreviation in texting or online chats for "I know." So the phrase means: "I know what you did last summer." This phrase is a well-known title of a 1997 horror movie and often refers to knowing someone's secret actions or activities from the previous summer.