It came top of a list drawn up in consultation with 1,000 linguists.
Ilunga means "a person who is ready to forgive any abuse for the first time, to tolerate it a second time, but never a third time".
It seems straightforward enough, but the 1,000 language experts identified it as the hardest word to translate.
In second place was shlimazl which is Yiddish for "a chronically unlucky person".
Third was Naa, used in the Kansai area of Japan to emphasise statements or agree with someone.
Although the definitions seem fairly precise, the problem is trying to convey the local references associated with such words, says Jurga Zilinskiene, head of Today Translations, which carried out the survey.
"Probably you can have a look at the dictionary and... find the meaning," she said. "But most importantly it's about cultural experiences and... cultural emphasis on words."Source: BBC News