I know what you mean
IKWUM, I wish I had chosen my major before my senior year too.
|IKR||I know, right?|
|IKWYM||I know what you mean|
|Gotcha||I get you|
|Say less||I understand|
Someone who's in sync with you may use IKWUM to mean, "I know what you mean." This acronym's usually sent in response to a statement to let you know the respondent feels you and/or has been there.
For example, if you tell your bro that you're heartbroken because your GF left you, he may respond with "IKWUM. My GF recently left me, too." The two of you can then share your experiences with each other and possibly come to the realization that you're both big jerks.