IPA: /ˌʌn.dɪsəʊˈeɪ.tɪd/
KK: /ʌnˌdɪsəʊˈeɪtɪd/
Not separated into smaller parts or groups; remaining in a whole or intact state.
The compound remained undissociated in the solution, maintaining its original structure.
Undissociated → It is formed from "un-" (meaning not) and "dissociatus" (from Latin, meaning to separate or disassociate). The word "undissociated" means not separated or not disassociated from something.
Think of 'not' ('un-') being combined with 'separated' ('dissociatus') — that's why undissociated means not separated.