A na-adjective meaning hasty or impatient; someone who is restless and dislikes waiting.
彼はせっかちな性格で、順番を待てない。
He has an impatient personality and can't wait his turn.
せっかちに話すと誤解を招くことがある。
Speaking hastily can sometimes cause misunderstandings.
彼女はせっかちだから、ゆっくり説明してあげてください。
She's impatient, so please explain it slowly to her.