To dig a long, narrow ditch. The workers trenched the field to lay down the irrigation pipes.
To cut into or carve out a trench in something. The soldiers trenched the hillside to create a defensive position.
To surround or fortify with trenches. The army trenched the camp to protect it from enemy attacks.
To encroach or infringe, usually followed by on or upon. The new regulations trench upon basic civil liberties.