You use this word to a younger brother or Sister.
"Nerd" for being unpopular and weird, and "lette" metioning that they are young. so in other words, a wierd young one.It's no word to take offense to, it just is a word brothers/sisters used to call there younger siblings to mess around.

Nerdlette: A common name for girls who don't like being call 'Nerds' because it sounds manly.

It's mostly used with a group of girls. Men mostly take offense when being called a Nerdlette.