Not long ago, a DPP "legislator" interrogated "Ministry of Education" why must grandparents from mother side be called "外"公 and "外"婆, and she claimed the appellations hint kind of sexisms and thus should be changed to the same as those from father side: grandpa and grandma only.

After his being dethroned, the last emperor of Han Dynasty and his queen, Cao Jie （曹節）were created the Duke and Duchess of Sanyang in Henan.  Queen Cao is the daughter of that famous Cao Cao, former the Duke of Wei（魏), so she asked her children and grandchildren to call her father as Wei Gong (魏公）and her mother as Wei Po (魏婆）。 Until now, people of some areas in central and northwestern China still call their grandparents from mother side as Wei Gong and Wei Po.  It is said through so many years the appellations have been transformed as Wai Gong (外公）and Wai Po（外婆）in some southern areas.

It takes linguists and folklorists to verify the origin of 外公 and 外婆; however, I think it may be true from the documented history.  Here is the story regarding Cao Jie: 

头条_爱铲史的猫_2022年03月19日_微头条-今日头条 (toutiao.com)   

PS That "legislator" is really a fusspot to raise a meaningless issue like this under the name of anti-sexism.  BTW, my two granddaughters simply call me and my wife as Gong Gong and Po Po.