If something is the rummiest, it is the strangest or most unusual.
The word 'rummiest' can make people think of funny or weird things.
Examples:
That is the rummiest hat I've ever seen.
The rummiest thing happened to me today.
She told the rummiest story about a talking dog.
The rummiest part of the movie was when the cat started singing.
The rummiest house in town is painted all the colors of the rainbow.
The rummiest thing about the alien was its three heads.