(noun)
a network of ditches cut into the ground that form an army's line, especially in France and Belgium during World War I, or a position/level of some activity that is casually likened to such a wartime scenario in the intensity of way it is carried out (often used as "the trenches" in both senses);
e.g. The two men fought together in the trenches during the world war. Many journalists are working in the trenches to provide coverage of the event.
(noun)
a long and narrow depression in the ocean bed, usually deep with steep sides;
e.g. There is an underwater trench in that section of the ocean.
(noun)
a long waterproof coat, usually double-breasted, and characterized by a wide belt and deep pockets, also called a trench coat;
e.g. She wore a trench made of fine dark gray material.
(verb)
to carve or cut something, especially to dig a long narrow ditch or a series of similar ditches in the ground;
e.g. He used a small spade to trench the flower bed.
(verb)
to border something or come very close to it, often to the point of entering into its territory, either literally or figuratively;
e.g. I don't want to trench on her time and occupy her with unimportant things.