Itchy Armpit is a land of the Barbaric Archipelago in How to Train Your Dragon 2.
Though no dragons have been seen, Hiccup suspected there might be Timberjacks, Whispering Deaths and perhaps even a Night Fury might be living in the forests, though the latter was probably to cheer up Toothless.
In How to Train Your Dragon 2
Hiccup and Toothless first encounter this new area after a crash-landing. Hiccup then proceeds to make a drawing of it on his map. Upon asking Toothless what they should name it, Toothless proceeds to scratch his armpit, giving Hiccup the idea to name the area "Itchy Armpit."
After Astrid catches up with them on Stormfly, they fly over the area to discover a large section of the forest had been burnt by fire, and so was a fortress, covered in gigantic ice crystals. This leads them to meeting Eret and his band of Dragon Trappers.