Fetching details from the dictionary...
very much; intensely; overwhelmingly; excessively; irresistibly; without any obvious reason
無性にラーメンが食べたくなった。
I got an overwhelming craving for ramen.
無性に泣きたい夜がある。
There are nights when I just want to cry for no reason.
無性に懐かしくなって故郷の写真を見返した。
I felt an inexplicable wave of nostalgia and looked back at photos of my hometown.