A combination of the word fest (party) and incest.
It refers to the act of incest, but as a positive term, like "fest" in "beerfest". The word's origin dates back to the Garden of Eden and the short period after, when the only way for the human race to pass it's genes on was, you guessed it, infest. It was a great time.
After all; nothing's like family.
"I had sex with my granddaughter last night"
"Sounds like a great party"
"No, it was more than that. It was infest!"
is the second album from the band Papa Roach. It was released in 2000 and features the songs Infest, Last Resort, and others. This album recieved a grammy nod.
Sex with someone with a familial relationship, but no blood relationship. Sometimes treated as incest in the law, but not as much of a taboo subject in commonsociety.
"Dude, she's not my mom! She just married my father."
"So that's pseudo-incest. Still illegal in this state."