How about "Extortion: Any offer of trade (t) by A to B, where A knows that the likely utility of B would be maximized if A had in advance treated (t) as certainly rejected."

In short extortion is any offer to you in which you could rationally wish you had clearly precommitted to reject it (and signalled such precommitment effectively), and A knows that.

Another example.

A and B share an apartment, and so far A has been doing all that household chores even though both A and B care almost equally about a clean house. (Maybe A cares slightly more, so that A's cleanliness threshold is always reached slightly before B's threshold, so that A ends up doing the chore every time.)

So one day A gives B an ultimatum: if they do not share household chores equally, A will simply go on strike.

B realizes, too late, that B should have effectively and convincingly pre-committed earlier to never doing household chores, since this way A would never be tempted to offer the ultimatum.

A is aware of all this and breathes a sigh of relief that he made his ultimatum before B made that pre-commitment.

By the above definition, A is an extortionist.