Definition 1:
A long expression used in reaction when something unbelievably crazy has just occurred / been witnessed / someone has found out about.
Definition 2:
Can also be used to describe the actions of an individual or group, who has just completed something of remarkable skill or achievement, and are worthy of comparison to Batmans skills.
Derived from the Robin ‘The Boy Wonders’ TV show catchphrase;
“Holy (insert object / person / event here) Batman!!”
Example 1:
Greg: “I just saw two trains crash together, and everybody onboard walked off alive!”
The more polite, reference using version of "Holy Shit." Rather than Holy Shit, it uses Holy Shitakii Batman, Shitakii an asian mushroom, Batman, a widely-popular DC Comics character.