It will always be used in a affectionate manner, since it is a pet name like honey
, etc... Although it CAN be a nickname for any girl, most likely spinning off from book-worm
, therefore most relevant when used for a girl who is slightly a brainiac, kinda cute in a mousey kind of way, and ideally wears glasses. Used in situations that cause seperation anxiety or just whenever someone wants to let someone know that they care about them and think they are special, most often said over the phone, or in person, seldom written. If used without forward consent however, it will comes off as either creepy or insulting.