Answer: The contempt he has for Trinculo.
Caliban uses this word in Act III, scene 2. The passage is the following:
<em> I say, by sorcery he got this isle;
</em>
<em>From me he got it. If thy greatness will </em>
<em>Revenge it on him, for I know thou darest, </em>
<em>But this </em><em><u>thing</u></em><em> dare not, </em>
In this scene, Caliban is plotting with Stephano to take the island away from Prospero, and rule it. He refers to Stephano as "thy greatness" and says that he believes he has what it takes to carry it out. However, he thinks little of Trinculo and does not think he would dare commit something like that. The contempt Caliban has for Trinculo is shown in his use of the word "thing."
