Modern English, from the surname.

The Oxford Names Companion [ONC] notes that Iagan, a diminutive of Aodh, Celtic "fire", gave rise to the surname Mac Iagain. This is Anglicized as MacKeegan and from this the surname Kegan was developed. Consequently it can be hypothesised that the forename Kegan is derived from this source, although ONC is silent on this recently popular forename.