IPA: /ɪmˈpjuːɪsənt/
KK: /ɪmˈpjuːɪsənt/
Lacking strength or power; weak and ineffective.
Despite his efforts, he felt impuissant in the face of such overwhelming challenges.
Impuissant is formed from "in-" (meaning not) and "puissant" (from Old French *puissant*, meaning powerful or strong). The word describes a state of being not powerful or lacking strength.
Think of 'not' ('in-') being combined with 'powerful' ('puissant') to remember that 'impuissant' means lacking power.