At 31:50 in the video, the professor said "It is under the six. It is absolutely impractical." And later at 31:54 he told a joke where he also mentioned "under the six". But I have searched the google and the Wikipedia, only to find that such a phrase doesn't exist. Thus I think I recognized the speech wrongly. Could anybody who can recognize it correctly(or has the knowledge background to recognize that ambiguous speech correctly) please tell me what it exactly is?
As Araucaria has already pointed out, the spoken words are:
It's n to the six, which is absolutely impractical.
"n to the the six" means "n raised to the power six". What the speaker is saying is that any solution would have a complexity of O(n6). Since solutions with complexity of less than O(n2) are generally sought for to be computationally viable, O(n6) is hardly acceptable.