Re: Waki ga nai
1: armpit; under one's arm; side; flank;
2: beside; close to; near; by;
3: aside; to the side; away; out of the way;
4: off-track; off-topic;
This is from one of my Japanese dictionaries. I take it that he using the first context (flank). Make sure you can't be out flanked, hence, no openings on your side.