what's-her-name; what's-her-face
Colloquial placeholder for a young female's name when the speaker cannot recall it or chooses not to say it. Often used in casual conversation or online. Similar to 何某 (なにがし) but specifically for young women.
I ran into what's-her-name at the station yesterday.
Did that what's-her-face girl show up late again?
Combination of 何 (なに, 'what') and the diminutive/honorific suffix ちゃん, used as a placeholder for a young female's name.