How to understand the meaning of 愛{いと}しさ I know that from adjective we can make adjectival noun like: {} -> - means _high_ and _hight_. But how to understand/translate the word ? I heard it in one of _C-ute_ songs.

I think I've figured out what you are asking about. Mental adjective + can refer to _**one's feeling**_ as well as _quality_ invokes the emotion. I'm not sure I'm able to tell their difference using English words, but you can paraphrase it with when it means the feeling. Maybe what you encountered was this kind of usage, where in this case = → ≈ "earnest love for you".