prefill的意思是在很渴的情况下先喝一杯。

用法解释：
When, in cases of extreme thirst, a person using a soft-drink fountain fills the beverage container, chugs it, and then fills it again before sitting down to eat.

A: Hey man, why did it take you so long to get a soda?
嘿，怎么拿饮料这么久？

B: I was prefilling my cup; I've been thirsty all day.
我先喝了一杯，口渴一天了。