This name does not appear on any map but is mentioned in the 1981 Árbók "Ódáðahraun" of Ferðafélag Íslands (page 97). A rough translation of the corresponding passage:
"[...] Then you come to a mountain slope or hill, which goes north from the Hvammsfjölls around the western slope of Kerlingardynga. It can actually be said that the mountains are a continuation of the mountain to the south. The part of this mountain slope that most attracts the tourist's attention is a single knoll that rises somewhat from the slope three km north of Hvammsfjöll. Its western slope is steep with rock belts at the top, but smooth and textured gray rock slides below. A greenish lick hits the slopes, and the same hue is on the egg-smooth patches of gravel that lie at the roots of the hill. On top of the hill on the east side you can see a crater(?). The hill does not have this name, but is marked with the height number 862 on maps. Due to the appearance of the knoll and the flats near its roots, the name Slétthöfði seems possible. On both sides of the hill, from the north and the south, ancient and rather turbulent lava flows fall down the slope. [...]"
Hint for hikers: At the steep western slope to the north and south of Slétthöfði you may find the last traces of snow patches in summer: