Is it possible to give a specific User the Power of Editing one of my Threads?

No

What do you mean give someone permissions to edit your thread... that can only be possible if he logs into your account... but you can't do that because account sharing is against the rules :/

If a specific user is staff or lmod of the board you can.
Also if the context is a clan thread then yes you can edit it by editing the clan page.

oh staff... alright, i thought he's talking about some normal members

He could be
He gave no context
So you have to guess what he's referring to

yes, i was referring to normal members, but i guess this has been answered then