Vsp. St 25 footnote: "Oath-breaking, lies and deception."

[jgnewman]

This is a pretty common interpretation, but I think it might be a misreading. Given that we only have Snorri's narrative to go by, Thor is absent for the entirety of the story until he is called upon to deal with the Builder who has decided to go on a rampage in Asgard. It's a pretty common theme for Thor to be completely absent from a story and then suddenly show up when invoked to deal with a problem-causing individual. I think it's less likely that "he seldom sits, when of such he learns" is a reference to him being angry over the oath-breaking and deception wrought by the Ease upon the ettin since his killing of said ettin is participatory to that oath-breaking. I think it's more likely that he gets mad when he learns of ettins "doing stuff" more generally.