If you're learning, or have ever learned, a foreign language, you'll have come across words that are difficult to pronounce. There are several words in English for such words, or describing such words. Some are in the OED, including:
jaw-breaker (noun): a word hard to pronounce, a word of many syllables
crack-jaw (adjective): fit to crack the jaws, difficult to pronounce.
crank (adjective): awkward or difficult to pronounce (a crack word). Obsolete, or Scottish.
Then, there are words in slang dictionaries, eg:
half-crown word: a difficult word, so-called because in the mid-19th century when the word was around half a crown was a lot of money.
sleeve-board: a word hard to pronounce. From tailors' slang
seventy-five cent word (US, late 19th century): a polysyllabic word.
Sesquipedalian is an adjective describing a long word, or a noun for the word itself, but a sesquipedalian word, though long, is not necessarily difficult to pronounce.