IPA: /ˌʌndərpərˈfɔːrmər/
KK: /ʌndərpərˈfɔrmər/
A person or thing that does not perform as well as expected or is below average in performance.
The team had to address the issues with the underperformer to improve overall results.
Underperformer → It is formed from "under-" (meaning below or beneath) and "perform" (from Latin "performare", meaning to carry out or accomplish) and "-er" (meaning a person who). The word describes a person who performs below the expected level or standard.
Think of someone who is 'below' ('under-') the expected level of 'accomplishment' ('perform') — that's why an underperformer is someone who does not meet performance expectations.