From A Wiki of Ice and Fire
|Spouse||An unknown Princess of House Targaryen|
|Book(s)|| The World of Ice and Fire (Mentioned)|
A Storm of Swords (Mentioned)
A Feast for Crows (Mentioned)
A Dance with Dragons (Mentioned)
He was an aged man when he wed Elaena Targaryen, he supposedly dropped dead on their wedding night after seeing her naked. Elaena though claims he did his duty before dying and she later conceived a child named Viserys Plumm. 
Many readers simply suspect that his wife gave birth to a child more than nine months after her husband's death and claimed the child was Ossifer's (in order for her son/daughter to lay claim to his lands/titles or to avoid the taint of being a bastard). His wife being a Targaryen Princess meant that few would dare counter the claim so this probably started the jest that Ossifer must have impregnated her from six feet under.
Tyrion speculates that Brown Ben Plumm is descended from a younger son of Viserys Plumm, and that rather than the "drop of dragon blood" Ben claims is in his family tree, there are "Two drops. That, or a cock six foot long." This suggests that Tyrion thinks Ossifer's posthumous child was fathered by another Targaryen.
Quotes about Ossifer
|“||There was some old Plumm in the Sunset Kingdoms who wed a dragon princess. My grandmama told me the tale. He lived in King Aegon's day.||”|
|“||This old Plumm was a lord, though, must have been a famous fellow in his day, the talk of all the land. The thing was, begging your royal pardon, he had himself a cock six foot long.||”|
|“||And Ossifer Plumm was much too dead, but that did not stop him fathering a child, did it?||”|