IPA: /ˌɪnɪˈfɪkəʃəsli/
KK: /ɪnˌɛfɪˈkeɪʃəsli/
In a way that does not produce the desired effect or result.
The new policy was implemented inefficaciously, leading to no improvement in productivity.
Inefficaciously → It is formed from "in-" (meaning not), "efficax" (from Latin, meaning effective), and "-ly" (meaning in a manner). The word describes doing something in a manner that is not effective or does not produce the desired result.
Think of 'not' ('in-') being combined with 'effective' ('efficax') to remember that 'inefficaciously' means doing something in a way that does not achieve effectiveness.