"想像の斜め上を行く" is used to describe things that are unexpected, or in a way different from what was expected. e.g. 想像の斜めを行く話題　topics that unexpected How to understand it? When people are thinking of "going up", most people will think of "↑ going up straightly". So when it goes up diagonally like↗︎, it will be considered as unexpected. Hope this would help :) 参考： http://matome.naver.jp/odai/2134267288949671401