A presupposition is anything that a native speaker of a language knows is part of the meaning of a sequence of that language, even if it doesn't appear on the surface of the sequence.
Example of a Pressuposition in the context of a Verbal Attack:
Consider the sentence,
- EVen JOHN could close THAT deal! (Capital letters emphasize intonation and focus of speaker).
Can you hear the presupposition? As a native speaker of English, you know that sentence presupposes 2 additional sentences, neither of which appears anywhere on the surface:
- The deal doesn't amount to much
- John doesn't amount to much, either
People that are not fluent speakers of English are unaware that the sentence is insulting to both John and the deal in question. How could they know? I repeat - because this is one of those situations where people are "too close to the forest to see the trees" - there is not one single negative word about either John or the deal in question. Nevertheless, because you are a native speaker you hear those negative presuppositions loud and clear.
(Copied from Chapter 6 of Success with the Gentle Art of Verbal Self-Defense)