expression, noun which may take the genitive case particle 'no'
completely normal; very ordinary; totally unremarkable
Emphasizes that something is utterly ordinary, with no special or unusual features. Often used in casual conversation.
彼の一日はごく普通だった。
His day was completely normal.
それはごく普通の出来事だ。
That's a very ordinary occurrence.
普通 alone means 'normal' or 'usual'; adding ごく intensifies it to 'completely normal' or 'utterly ordinary'.
Combination of the adverb ごく (極, 'extremely') and the noun 普通 ('normal, ordinary'). The exact historical derivation is straightforward.