IPA: /rɪˈɡeɪnər/
KK: /rɪˈɡeɪnər/
A person who recovers or gets back something they lost, especially in terms of body weight.
After her diet, she became a regainer and put on the weight she had lost.
Regainer → It is formed from "re-" (meaning again) and "gain" (from Old French *gaigner*, meaning to earn or acquire) and "-er" (meaning a person who). The word "regainer" means a person who earns or acquires something again.
Think of someone who is 'earning' ('gain') something 'again' ('re-') — that's why a regainer is someone who regains what they had.