Something that is farttastic smells bad. If it's not a tangible thing, but you think it would smell bad if it was real, that would also be farttastic.
In Spanish, the word is farttastico.
rachel: imogen and spek should come back.
israel: yeah, them being gone is totally farttastic.
child: mom, what are you cooking?
mother: my family's secret recipe, doesn't it smell great?
child: no, it's totally farttastic!