Dagnir's name comes from the Elvish root dag- 'slay'. This is the same word as appears in Túrin's title of Dagnir Glaurunga, which is translated as 'Bane of Glaurung', but might be rendered as 'slayer of Glaurung'. 'Slayer' might, incidentally, seem rather an unlikely choice of a birth-name, so perhaps we're to take it that Dagnir acquired this name as a later epithet during his time as a desperate outlaw in Ladros.