From Book I Chapter 1 of The Fellowship of the Ring by J. R. R. Tolkien:
Mr. Drogo married poor Miss Primula Brandybuck. She was our Mr. Bilbo's first cousin on the mother's side (her mother being the youngest of the Old Took's daughters); and Mr. Drogo was his second cousin. So Mr. Frodo is his first and second cousin, once removed either way, as the saying is, if you follow me.
What is the meaning of the phrase in bold?