Something hard to say

• What Peter Piper did, for example, is hard to say

Wiktionary Word definitions in Wiktionary
n. A phrase that is deliberately designed to be difficult to say correctly, usually because of varying combinations of similar phonemes.

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1875, in reference to an awkward sentence, 1892 of a deliberately difficult-to-say phrase, from tongue (n.) + agent noun from twist (v.). The first one called by the name is "Miss Smith's fish-sauce shop."

Wikipedia Word definitions in Wikipedia
A tongue-twister is a phrase that is designed to be difficult to articulate properly, and can be used as a type of spoken (or sung) word game . Some tongue-twisters produce results which are humorous (or humorously vulgar) when they are mispronounced, while ...