I'm not sure this is what you were referring to but my mother used an egg in coffee. The reason (I thought) was to gather the grounds so there was less in the poured cup.
The technique she used was to place dry grounds in the bottom of a coffee pot, add the egg (mix), pour in boiling water, let come to just a boil again. Without the egg you'd get quite a bit of grounds in the pour. With the egg there was much less.