“일편단심 애정공세지” = 일편단심 애정 공세이지. = It is unwavering (agressive) attempts to win your heart (in romance/love). 일편단심 = (idiom) unwavering love, loyalty, etc. (literally "a piece(일펴 ㄴ) of red heart(단심)") 애정 공세 = love(애정) 공세(agression) = aggressive move to win someone's heart. -(이)지 = 이다 + -지 (a sentence ending expressing an idea one is reasonably sure of to let the other person know or to get confirmation).