The term originates from a theoretical discussion in the Talmud regarding a Jew who was kidnapped by Gentiles as an infant and therefore sinned inadvertently for lack of halakhic knowledge.
This is a case where the individual in question is not responsible for his actions and sins due to his being raised in a place or situation where the Jewish law is unknown to him.
In Shabbat 68b, there is a dispute between Abba Arikha and Samuel of Nehardea on one side, and Yochanan bar Nafcha and Shimon ben Lakish on the other, regarding in what type of situations a tinok shenishba (or a convert who was similarly raised among gentiles) is responsible for punishment and/or repentance along with the offering of animal sacrifices in the Holy Temple upon transgression of the laws of Shabbat and their subsequent return to Judaism.
The notion that unaffiliated and unobservant Jews are unwitting sinners who should be taught the Jewish laws and customs and welcomed into the Torah community is the basis for the many Orthodox Judaism outreach organisations (Kiruv) that exist in the modern era, including Chabad, Aish Hatorah, Ohr Somayach and Gateways.
The Baal Shem Tov (1698–1760), founder of the revivalist Hasidic movement, brought a mystical soul-dimension to the traditional Talmudic notions of the tinok shenishba, and the am ha'aretz (uneducated-boorish-rustic Jews).