Ever since dad got remarried, his new wife became a real stepmonster who won't even let me go out on weekends.

stepmonster 是由 step（继）和 monster（怪物）组成的复合词，step 表示"继"的意思，monster 本义是"怪物"，这里用作比喻。 stepmonster 是贬义词，用来形容"恶毒的继母"，带有夸张和厌恶的语气，把继母比作怪物。