The Spanish Finger is a gesture given to someone who has been NAUGHTY. It is equivalent to smirking while shaking one's head. To give the Spanish finger: lift your right arm, hold only your right index finger in the air, and move slightly right to left (this may be accompanied by a flirty smile if desired).
You are at a bar and a young, cocky but still cute international playboy comes up and tells you that he is taking you home later. Even though you are planning on going home with him as well, you give him The Spanish Finger to keep him on his toes. It is usually followed by a smart ass remark.