A single word for "hard to pronounce"

The colloquial word, 'jaw-breaker', means "hard to pronounce":

jaw-breaker, n.
colloq.
1. A word hard to pronounce ....

Several other words meaning "difficult to pronounce" are in use:

break-teeth adj. (also break-tooth) difficult to pronounce.

'Break-tooth' has a distinguished pedigree, including use by Sir Walter Scott (1827).

crack-jaw, adj.
....
Fit to crack the jaws; difficult to pronounce. Also transf.

'Crack-jaw' has a history of use that is even more illustrious than 'break-tooth', including uses by Disreali (1827) and Beerbohm (1920).

'Cramp' has its own charm, but is likely to lift an eyebrow or two when used:

cramp, adj.
1. a. Difficult to make out, understand, or decipher; crabbed.
cramp word: a word difficult to pronounce or understand.

(All from the OED Online.)

The first three of these words will very likely be easy to understand, even for people not familiar with their use and meaning. The fourth may require explanation of its meaning in most company, unless it is used in the phrase "cramp word", which might make the need to explain its meaning less likely. None of them are designated as obsolete or rare in the OED.

A word (or phrase) that's difficult to pronounce is a tongue-twister.