This word is a combination of chill and itis.
Itis, for the most part, results in sleep. While Chillitis results in lazyness and minimal activities.
Nigga B: You feelin' you gonna pass out? There's a bed upstairs.
Nigga A: Naw man, just, you know... chillin', not really tired...
Nigga B: That ain't no ordinary itis, that's what we call "Chillitis".