im
Explanation
This comic is a linguistics joke about the word "immediately." A professor-like figure explains the etymological fun fact that "immediately" derives from the roots "im-" (meaning "not") and "mediate" (meaning "in the middle"), giving the sense that something will be done without delay by other activities -- i.e., with nothing mediating between now and the action.
His student Steve responds, "This is a very important analysis." The professor then says he'll have it done "im-mediately" -- but the punchline twist is that the student walks away saying he'll have it done "mediately," meaning he will put something in the middle (i.e., delay). By removing the "im-" prefix, the student has inverted the word to mean the opposite: he'll get to it eventually, with plenty of other things in between.
The humor works on two levels: it's a straightforward wordplay joke about Latin prefixes, and it also captures the universal experience of someone acknowledging that something is important but then clearly planning to procrastinate on it.