This term was coined by a great artist. It is used in describing something that is unfair, jankety, smelly, bullshitty, unpleasant, unfortunate, etc. In rare cases boomsquash can also be a triumphant term (replace beyaaawhh!, or booyah
"hey bro we're out of beer."
or --"thats fucking boomsquash son."
or --"Your boomsquash ass drank it all."