There's somebody sitting in my seat.

somebody Determiner

— Somebody is a person, but you don't know which person you mean. I think I saw somebody outside. Somebody keeps leaving the lights on.

somebody Noun

— A somebody is a person who is famous or important.

sitting

— noun the act of assuming or maintaining a seated position he read the mystery at one sitting — noun (photography) the act of assuming a certain position (as for a photograph or portrait) he wanted his portrait painted but couldn't spare time for the sitting — adjective (= seated) (of persons) having the torso erect and legs bent with the body supported on the buttocks the seated Madonna the audience remained seated — adjective not moving and therefore easy to attack a sitting target — noun a session as of a legislature or court — noun (= seance) a meeting of spiritualists the seance was held in the medium's parlor

in Preposition

— Used to show that something is inside something else. The cat is in the box. — Used to show that someone is at home, or is available. Is John in? The Doctor is now in. — Used to show movement towards the inside. The rain came in through the window.

my Pronoun

— My things are things that belong to me. Is this my book, or is it yours?

seat Noun

— A seat is something to sit on. Pull up a seat and sit down. — The seat is a part of the clothes covering the buttocks. The seat of your pants is muddy. — Seat is where something is, at the centre of things. The town was the county seat, where the county council met. Canberra is the seat of the Australian Government.

seat Verb

— To seat is to help someone sit down. Follow me. I will seat you near the stage. I will pull out your chair to seat you.