The Doctor's time travel machine (from
Doctor Who), which is more spacious on the inside than it appears from the outside. In this conversation, it seems to have become a word to describe anything that bears the same characteristic.
p.s. It occurs to me there are three main theories: compression, teleportation and TARDISion. The first one strikes me as being the most likely, because the Stranger talks about (I forget which bounty) being put in a tiny tin can. Assuming he understands how the can works, he implies that the bounties go inside, and that the interior is tiny.
And also, a quick spot of Wikipediaing tells me that “the word “TARDIS” has been used to describe anything that seems bigger on the inside than on the outside.”