Instructions
Launch "Counter-Strike: Source."
Click on "Options." Select "Keyboard," "Advanced." Check the box next to "Enable Developer Console." This is a window that allows you to alter the game's settings.
Open the developer console by pressing the "~" key.
Type 'bind X "drop,"' without the single quotes, in the developer console. Press "Enter" on your keyboard. This will bind the "drop weapon" command to the X key on your keyboard. If you want to bind it to a different key, replace "X" with your choice. You can now automatically drop your weapon to allow a friend to pick it up by pressing the key you chose.