Is 도둑 a pure Korean word?

Answer: No.

Pure Korean words are essentially words that Koreans were using before they started borrowing words from the Chinese. In Korean, pure Korean words are sometimes referred to as 토박이말 or 고유어 (固有語), which translates in English as indigenous language or native tongue. They are words such as  아버지 (father), 어머니 (mother), 아들 (son), 딸 (daughter), 하늘 (sky), and 땅 (earth / land /ground). They are words that usually describe family, farming, and the everyday things that were familiar to Koreans about 2000 years ago. They are Korea's oldest words, the words that were so important to Koreans that they have survived the Chinese word invasion that started a little over 1500 years ago.

Probably because pure Korean words are Korea's oldest words, Koreans seem to have more of an emotional attachment to them than they do to words borrowed from other countries, including China. In fact, Koreans usually seem to know if a word is pure Korean just from the sound of it. However, some words that many Koreans might think are pure Korean are actually words that have evolved from borrowed words. One of those words is the word 도둑, which means thief.

I have read that 도둑 evolved from the Chinese word for thief, which is 盜賊 (도적). I do not know how 도둑 evolved from 도적, but the evolution has somehow made 도둑 sound like it is pure Korean, even to a non-Korean like me.

There are other pure-Korean sounding words similar to 도둑, but I am not going to list them here because, honestly, I have other things to do and want to get this post out of the way. However, I will end the post by referring back to the Korean words for indigenous language: 토박이말 and 고유어 (固有語).

The word 고유어 (固有語) is obviously not a pure Korean word since it has Chinese characters associated with it, but even 토박이말 is not completely pure Korean since the 토 syllable in 토박이말 comes from the Chinese character for land, 土 (토). The word 토박이말 literally translates as words that are nailed to the land.