walking on eggshells; treating with extreme care
Describes treating someone or something very delicately, as if touching a swollen boil, to avoid causing pain or trouble. Often used with 扱う or 接する.
He's quick to anger, so I treat him like he's made of glass.
The boss is in a bad mood, so I approached him today as if walking on eggshells.
おっかなびっくり describes doing something fearfully or nervously, often physically, whereas 腫れ物に触るよう focuses on interpersonal delicacy and careful handling.
Literally 'like touching a swollen boil,' this idiom compares the careful handling of a painful swelling to the delicate treatment of a sensitive person or situation.