most of all; extremely; supremely
Adverbial phrase emphasizing the highest degree; often used before adjectives or verbs to mean 'more than anything' or 'to the utmost'. Slightly formal or emphatic.
I am supremely happy.
It is an extremely important issue.
Standard kanji-kana spelling.
Variant using the kanji 無 for なく; less common but still encountered.
From この上 (kono ue, 'above this') + も (mo, 'even') + なく (naku, adverbial form of ない 'not'), literally 'not even above this', i.e., 'nothing surpasses this'.