The Rule of 'ish' applies to when someone is scheduling an activity's time. The person who designates the time with an 'ish' to the end implies that the activity may start 15 minutes before of after the given time. Failure to be ready during the 15 minute 'ish' window will result in sitting in the back seat of the car or (if no car is available) you have the right to make fun of the loser who makes you wait.
The Rule of 'ish'
Person 1- Hey we'll pick you up at 12ish
Person 2- Ok
Person 1- Ok were here
Person 2- But im not ready
Person 1- I guess thats backseat for you...FAGGIT!