The word "注射" is formed by combining the character "注", meaning to pour or inject, which incorporates the water radical (氵) to imply liquid and "主" to suggest directed action, with the character "射", meaning to shoot or launch, derived from elements representing body (身) and hand (寸) to convey projection. Together, they create a compound term where "注" conveys the transfer of liquid and "射" conveys the forceful action, resulting in the meaning "to inject", as in a medical injection.