tongue twister

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A word or group of words difficult to articulate rapidly, usually because of a succession of similar consonantal sounds, as in Shall she sell seashells?
• noun Something difficult to pronounce.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Alternative form of tongue-twister.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an expression that is difficult to articulate clearly