A compound of what is their name.

whatstheirname (plural whatstheirnames)

(informal) A person whose name is either unknown or forgotten.
- 2018, Philipp Winkler, Hooligan‎:
  Many old names, hairstyles, and mustaches that I'd already forgotten: Mirko Votava, Tom Dooley, Vladimir Beschastnych, Manni Bender, Heiko Scholz, Alain Sutter, and whatstheirname
- 1973, Records Management Quarterly‎, page 9:
  Maybe now you can see the outstanding job they are doing for you as you call for “that letter about the pipe joints rom Whatstheirname's warehouse,"

English