one of tadashi yamaguchi's many phrases lovingly spoken to kei tsukishima, who he's definetely a LOT sus for.. 😌 it means "i'm sorry, tsukki!" a common quote heard among haikyuu stans!
Pettan alone means flat. For example, the suffix -ko means "girl" (such as in meganeko, megane = glasses so glasses girl) so pettanko means a flat (chested) girl.