That You May Swear

[Mher]

Friends,

What does the underlined clause mean? I could not find a suitable meaning of "swear" that would fit the context. This is from Poe. The link is as follows: https://poestories.com/read/systemoftarr.

"Oh, a very long time, indeed — a month certainly — how much longer I can't precisely say. In the mean time,
the lunatics had a jolly season of it — that you may swear. They doffed their own shabby clothes, and made free
with the family wardrobe and jewels. The cellars of the chateau were well stocked with wine; and these madmen
are just the devils that know how to drink it. They lived well, I can tell you."

I guess the speaker means here that the one whom he addresses can be sure about that. Right?

[Raymott]

Yes, it means that the hearer could swear that the lunatics had a jolly season of it, and he would not be wrong.