Wordnet 3.0
NOUN (2)
1. 
 a buzzing or whizzing sound; 
- Example: "a nasty zizz in the engine"2. 
 a nap; 
- Example: "Arthur's taking a short zizz"
WordNet (r) 3.0 (2006):
zizz
    n 1: a buzzing or whizzing sound; "a nasty zizz in the engine"
    2: a nap; "Arthur's taking a short zizz"