I have come across this surname Stillborn before and did research, but I never figure out its origin because of its rarity, until today. Because of Viatrix question, I did research again and this time I might have the answer to your question. A much more common but still rare variant is Stilborn. This is a Canadian surname, also found in the UK, but barely. With my research, I think this surname Stilborn is a variant of English surname Silburn. Variants include Selburn, Silbourne, Silborn, Selborn, Sylborn, Selbourne, Cilborn and others. In ancestry.com, there are a couple Stilborn bearers in Yorkshire, and perhaps this is where they are from. (http://www.ancestry.com/facts/Stilborn-family-history-uk.ashx
) The surname Silburn is also most concentrated in Yorkshire, and in the county Suffolk and Northamptonshire. (http://www.ancestry.com/facts/Silburn-family-history-uk.ashx
) Perhaps there is a connection between the Silburn and the Stilborn families. In surnamedb.com, the surname Silburn came from a village called Selborne in Hampshire. (http://www.surnamedb.com/Surname/Silburn
) The most concentrated places of this surname Silburn is in Yorkshire and Suffolk. Barely any Silburn are found in Hampshire, so perhaps unlikely that the Silburns originated from Hampshire. Another close variant I found is English surname Seaborn, an Essex surname. This probably explains the the Silburn of Suffolks, being a variant of Seaborn, also as Seaburn. Another variant surname I found of Seaborn is Seabourne, but this surname Seabourne is mostly concestrated in Wales, in the counties Gloucestershire, Monmouthshire, and Hertfordshire. Perhaps the Silburns of Yorkshire is of different origin, and that the surname Stilborn is a variant of the Silburns of Yorkshire. Perhaps from a lost village in Yorkshire, or perhaps and English form of a French surname. I'm think its of French origin...
The meaning of English surname Seaborn can be found here (http://www.ancestry.com/facts/Seaborn-family-history-uk.ashx