Interjection, similar to holy cow!
From the classic, campy Batman: The Movie starring Adam
West. Robin (Burt Ward) had a habit of making strange exclamations (Holy
Polaris, Holy Sardine, Holy Captain Nemo... you get the idea.
At one point in the movie, Batman is attacked by--you guessed it--an inflatable shark, against which he must defend himself whilst the faithful Robin runs to get the shark-repellent bat
spray.
Should be exclaimed loudly and dramatically, for maximum campy effect.