IPA: /ˌɪnɪˈfɪʃəntli/
KK: /ɪnɪˈfɪʃəntli/
In a way that does not use resources, time, or effort effectively; producing less than the best result.
The team worked inefficiently, causing delays in the project.
Inefficiently → It is formed from "in-" (meaning not) and "efficient" (from Latin "efficiens", meaning accomplishing or producing a desired effect) and the suffix "-ly" (meaning in a manner). The word "inefficiently" means in a manner that does not accomplish or produce the desired effect.
Think of 'not' ('in-') being combined with 'accomplishing' ('efficiens') to remember that 'inefficiently' means doing something in a way that does not achieve the desired result.