IPA: /ˌʌndɪˈkɔːreɪtɪd/
KK: /ʌndɪˈkɔrɪtɪd/
Not having any decorations or embellishments; plain in appearance.
The room was undecorated, with bare walls and no furniture.
Undecorated → It is formed from "un-" (meaning not) and "decorated" (from Latin "decorare", meaning to adorn or embellish). The word "undecorated" means not adorned or lacking decoration.
Think of 'not' ('un-') having any 'adornment' ('decorare') — that's why undecorated means lacking decoration.