I did a quick Google search on Laswell, and it seems that it is an Anglicised version of the French surname Lascelles. I couldn't find a meaning for it though.
This page is an advertisement for the Lascelles/Laswell family crest, with a small amount of information:
http://www.houseofnames.com/coatofarms_details.asp?sId=&s=laswell
This may be a good place to start looking. Be cautious though - such sites aren't always accurate and are sometimes complete fabrications. However there are a few good genealogy resources out there, and they may be able to help you confirm that Laswell came from Lascelle in your case. Another place to try is Behind The Name's surnames message board:
http://surnames.behindthename.com/surboard.html
Ask them and they may be able to help.
Good luck :-)