Wikipedia
Yestonians was a derogatory epithet for historically ethnic Estonians brought from Russia to Estonia after World War II to staff the political structures of Soviet Estonia with cadres loyal to Moscow. While their ethnicity was Estonian by birth, they grew up in the Russian/ Soviet environment, which meant that for many, the primary language was not Estonian, but Russian (the ethnic mother language was a second language), which in turn made them prone to apply Russian-language pronunciation rules on Estonian-language texts that they were to publicly read out in speeches.
The term thus relates to and derides the heavy Russian accent of these people and their practical inability to speak Estonian. To alleviate this, they inevitably read their speeches from paper, and words for Estonians were mispronounced from eestlased to jeestlased'' [yeestlɑsed''], serving as the origin of the epithet. This and the Russian accent were so noticeable, that some high-ranking politicians, such as Karl Vaino, avoided giving speeches in Estonian.
While some of them tried to re-acculturize, such as Ivan Kebin, who renamed himself to Johannes Käbin and notably improved his command of Estonian, most others remained Russian by culture and language.