So in Game Of Thrones Grand Maester Pycelle says to Eddard that Jon Arryn was rambling a lot before he died but he kept repeating "The seed is strong." Now I have seen all four seasons all the way through and I still don't know what it means. Lysa Arryn says that he was talking about Robin but she is pretty crazy so I'm just wondering if I have missed something or if it is explained in the books.
In the the first series Eddard Stark discovers that
Cersei's children are not king Robert's, but Jamie's.
He does this by using the same book that Jon Arryn uses to research the genealogy of the Baratheons through the ages.
Eddard discovers that every Baratheon child for centuries has black hair. This is proven when he searches out all of King Robert's bastards and finds that they too have black hair.
The reason he finds this out is because he's following Jon Arryn's footsteps before he died, so we can conclude that Jon Arryn discovered the same secret and was murdered for it.
Genealogically, people tend to have certain traits because certain characteristics of a human are more likely to occur because of dominant genes.
However before scientific breakthroughs people would have assumed it's because of the person, rather than the genes themselves, why this occurred.
So when Jon Arryn was repeating "The seed is strong", he was trying to reveal the secret he had discovered before he died.
In the throes of death when he was poisoned he wouldn't have been able to give a full description about his findings, so he hoped that someone would understand from these words what he had found. Which Eddard eventually did.
He's talking about the Baratheon seed. It's strong so all Baratheons have black hair, regardless of who they breed with. All of Robert's bastards have black hair. All Baratheons going back years have black hair. Since the seed is strong, Robert's "children" by Cersei couldn't possibly be his, because they have blonde hair.