Sheldon: Like we are on a ship, coming from Africa to America.
Amy: Sheldon, that’s completely inappropriate. You can’t keep comparing yourself to a slave.
Sheldon: Yes, Miss Amy.
Leonard: I can’t believe you’re married to that idiot.
Penny: Ugh, would you stop? We just did it as a goof.
Leonard: Well, a goof or not, you’re actually married. You need to get this taken care of.
Penny: I will. Why are you making this such a big deal?
Sheldon: Oh, oh, oh, I have a reason. It could be because you said yes to marrying Zack, but every time Leonard’s proposed, he’s gotten resounding no. Yeah, that’s just *off the top of my head.
*off the top of your head 意思就是一件事没有经过认真思考或核实、仅仅靠推测或是记忆就说了出来。可以用来表达“一时想起、大概估计”的意思。例如： I can't remember the name off the top of my head, but I can look it up for you. （那个名字我一时想不起来，但是可以帮你查查。）
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