IPA: /ˌʌn.rɪˈhɜːrst/
KK: /ʌnriˈhɜrst/
Something that is done without preparation or practice, often spontaneous or improvised.
The speaker delivered an unrehearsed speech that captivated the audience.
Unrehearsed → It is formed from "un-" (meaning not) and "rehearsed" (from Old French *rehercer*, meaning to practice or repeat). The word "unrehearsed" means not practiced or prepared in advance.
Think of something that is 'not' ('un-') 'practiced' ('rehearsed') — that's why unrehearsed means it hasn't been prepared beforehand.