hit someone for six
hit (one) for six
To upset or overwhelm someone, especially suddenly or unexpectedly. Primarily heard in UK. News of my father's sudden death completely hit me for six.
hit (or knock) someone for sixaffect someone very severely; utterly overwhelm someone. British informal
In this expression, six stands for six runs, referring to a hit in cricket which sends the ball clear over the boundary of the ground for a score of six runs.