Commonly used in the underground rave scene in India and Israel.....'Shankar' is one of the many names for Shiva, one the 3 main deities in the Indian pantheon.
His dance is the rhythm of the world, the beat of the heart, the cycle of day and night. He lives in the Himalayas, and lies in the mountains smoking cannabis and his hair flows down from the mountains as the holy river Ganges.
His worshippers include "sadhus"; wandering mendicants, whose worship includes smoking chillums of hash/ganja (Marijuana), so they became very popular with the wandering freak set India.
bom, or boom, or bem, or bam is a sound --- a sharp sound.
Boom Shiva: it is said by sadhus before they light the chillum to: sanctify it Shiva, let Shiva know the chillum is being smoked, let other sadhus et al know to come share a chillum
Loosely translated as: OH Shiva... I`m smoking a chillum, hey you sadhus, come and share it with me"; Also used in the rave scene while lighting a chillum (or even a spliff/joint at times) and passing it around.