Plein comme un oeuf

Literally translated as, “full as an egg,” it is a colloquial simile applied to a thing or a place that’s completely full.  Close English equivalents would be “filled to the brim” or “packed to the gills.”

What is important for an English speaker to know is that the expression can’t be applied to a person.  “Full” here is not to be understood as having a full stomach, because the French adjective plein is not used in that sense.  You would say instead : je suis rassasié - I've had enough (food) -  or je n'en peux plus - I can't have any more (food.)

Origin

This expression has been around since the 17th century at least. The reasoning behind it is easy to understand: the white and yolk of an egg fill it entirely, and it would be impossible to squeeze anything more inside the shell.

Nitpickers in the crowd will argue that an air pocket develops inside the egg as it ages until it’s not so full anymore - this is caused by the gradual evaporation of its water content through the porous shell.  A more accurate version of the expression might then be plein comme un oeuf fraîchement pondu (full as a freshly laid egg), but we're not sure how well that would take.

Examples

Je peux mettre mes chaussures dans ta valise ? La mienne est pleine comme un œuf.
Can I put my shoes in your suitcase? Mine is packed to the gills.

Ce bus est plein comme un oeuf.  Attendons le prochain.
This bus is packed.  Let's wait for the next one.