The Zohar, Ra'aya M'hemnah, in Parshas Tzav says that Teiku means it will always stand as a question, as it stands for Tikun minus the nun sha'arei Binah (the 50 [=Nun] gates of understanding). This is used as a signal that this question comes from the klipos (shells) and could not have an answer because the halacha it is addressing has an element of gezeirah (divine decree) to it and not fully understandable by Man.
It goes on to say that Eliyahu will answer other ones that do not end in "Teiku". This contradicts the tradition that YS quotes. As usual, this Zohar needs a Rebbi to explain it.