*treu(k)-

The core idea of the root was:

pushing, squeezing, grinding

There might be a connection to the root of the verb drehen (to turn), but that’s not certain.
The sense of threatening that we have in German and English is based on the notion of mental pressure, mental “pushing”.

Here’s  list of notable English members of this family:

• threat, to threaten (“pushing mentally”)
• thrust (“pushing forward”)
• intrude (“thrust in, force in”)
• protrude (“thrust out, push out”)
• abstruse (“pushed away”)