IPA: /ʌnˈtjuːnd/
KK: /ʌnˈtund/
Not adjusted to the correct pitch or frequency.
The piano sounded off because it was untuned.
Untuned → It is formed from "un-" (meaning not) and "tuned" (from Old English "tunian", meaning to adjust or arrange). The word "untuned" means not adjusted or arranged, particularly in the context of musical instruments.
Think of 'not' ('un-') being applied to something that has been 'adjusted' ('tuned') — that's why untuned means it hasn't been adjusted.