Abarim Publications' online Biblical Greek Dictionary
The adjective νοθος (nothos) denotes slave-born, illegitimate or spurious offspring or artistic productions (such as plagiarized writings). Our adjective shows up all over the classics (even in Latin, as nothus) and is most notably used to describe a bastard child, or a child from a citizen father and a foreign mother.
It frequently occurs juxtaposed with the noun υιος (huios), which is the common word for son, but in the legitimate sense: an accepted son, a son by rank and status not mere biology. As such it appears in its only Biblical occurrence, namely in Hebrews 12:8, where Paul declares that an undisciplined person is a νοθος (nothos) and not a υιος (huios).
Where our word comes from isn't wholly clear, but the Latin noun notia denotes a kind of gem that was said to fall from the sky along with rain. The Greek word νοτια (notia) indeed means damp or moisture, and the adjective νοτιος (notios) means moist or damp or "bringing rain". This is remarkable because the Greek word for actual rain is υω (huo), and is not unlike the noun υιος (huios), for son.