Wiktionary
whatsername
n. (context colloquial English) A placeholder for the name of a woman whose name the speaker can not bring to mind.
Usage examples of "whatsername".
Not that it matters if the whole lot of them, Belburga, Lily, Ruby, and that other flowery brat, whatsername, are boiling up a kettle to cook me in right now.
Call that girl: well, call her, and have whatsername send up some nosh.
My glasses have steamed up a bit, so I can't see Kate Bush's face amongst them, just a glimpse of that chippy, dark-haired Jewish girl, Rebecca whatsername, but it's too late to stop dancing now, because James Brown is asking me to shake my moneymaker, shake my moneymaker, and I have to think for a minute because I'm not sure what my moneymaker is specifically.