The full form of imk is in my knowledge.
The acronym “imk” stands for the phrase “in my knowledge.” It is used when you want to state that you are pretty certain of something, but you are not 100% sure you are correct.

Origin
As seems to be the case with most forms of internet and text slang that is used today, there is no known information regarding the origin of the acronym “imk.”

Other Meanings
Although this “IMK” has been the associated acronym for some companies and businesses, there is only one other suggested phrase that this acronym may be associated with. Some say the acronym “imk” can stand for the phrase “I must know.” However, this is not a popular opinion. Just in case, it is important to keep this in mind and read in context to know which meaning is implied by the use of “imk.”

Example Conversations
A text exchange between friends.

Friend 1: IMK, the best way to go to the mall is to take I-95 to exit4.
Friend 2: No way! That highway is always crowded. It is better to go the back way, even if it takes longer.