Scottish: from an Anglicized form of the Gaelic personal name Fearsithe, composed of the elements fear ‘man’ + sith ‘peace’. Some early forms with prepositions, as for example William de Fersith (Edinburgh 1365), seem to point to an alternative origin as a habitational name, but no place name of suitable form is known. The spelling Forsythe is associated chiefly with northern Ireland.

Dictionary of American Family Names, Oxford University Press, ISBN 0-19-508137-4

Forsythe Family Crest and History

Where did the Scottish Forsyth family come from? What is the Scottish Forsyth family crest and coat of arms? When did the Forsyth family first arrive in the United States? Where did the various branches of the family go? What is the Forsyth family history?
The Picts of ancient Scotland were the tribe of the ancestors of the Forsyth family.  The name Forsyth is derived from the old Gaelic personal name Fearsithe, which means man of peace.
However, some recorded examples of the surname Forsyth suggest that it is occasionally a local name derived from residence a place named Forsythe.

Forsythe Early Origins
Before the first dictionaries appeared in the last few hundred years, scribes spelled according to sound.
Spelling variations are common among Scottish names.  Forsyth has been spelled Forsyth, Forsythe, Forseyth, Forsy, Foursides and others.
First found in Stirlingshire, where they held a family seat from very ancient times, some say well before the Norman Conquest and the arrival of Duke William at Hastings in 1066 A.D.