Wordnet 3.0
ADJECTIVE (2)
1. 
 impossible or difficult to pronounce correctly; 
2. 
 very difficult to pronounce correctly; 
- Example: "an unpronounceable foreign word"- Example: "unutterable consonant clusters"[syn: unpronounceable, unutterable]
The Collaborative International Dictionary of English v.0.48:
Unpronounceable \Unpronounceable\
   See pronounceable.
WordNet (r) 3.0 (2006):
unpronounceable
    adj 1: impossible or difficult to pronounce correctly [ant:
           pronounceable]
    2: very difficult to pronounce correctly; "an unpronounceable
       foreign word"; "unutterable consonant clusters" [syn:
       unpronounceable, unutterable]