slip one's mind 忘记；忘了；忘记了

【例句】
I forgot I'd arranged to meet Richard last night -- it completely slipped my mind.
我忘记昨晚安排好与理查德见面了——我把这事完全忘了。

I'm sorry I didn't call you back sooner, it totally slipped my mind.
抱歉，我没有早点给你回电话，我完全忘了。