There are some spam issues in the comments, so I had to disable comment editing for a short time. It'll be back once the spam bots are gone. FYI... they leave a comment with "great" and then edit in the spam stuff later, to get around the filter.
(This root might be a variation of the root "*ter-" which was about turning. Notable English members of this family are "threat", "thrust", "intrude" and other words from the Latin branch of "trude".)