Janvrin is derived from Old French jenvre 'youngest (child in the family)', from Latin juvenior/junior (see
http://www.geneanet.org/onomastique/index.php3?nom=janvrin). The surname is found mainly in northwestern France as shown at
http://www.geopatronyme.com/cgi-bin/carte/nomcarte.cgi?nom=janvrin&submit=Valider&client=cdip but the North American Janvrins come from Jersey in the Channel Islands as you can see at
http://awt.ancestry.com/cgi-bin/igm.cgi?op=GET&db=flybaby2&id=I386