I've been watching a movie that one said:
How a halfwit like you pull that off?
and the other said as the answer:
Guess the halfwit's still got some juice.
I searched but didn't get any meaning for "juice" that makes sense here.
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It's sort of mixing metaphors, emphasis on the "sort of." A "halfwit" is the same thing as a "dimwit" or a "dimbulb," all terms for a stupid person.
For whatever reason, light is a common metaphor for intelligence, more light representing higher intelligence and less light representing low intelligence, so adjectives like "bright" and "brilliant," which describe something as shining much light, are used to describe someone as having high intelligence, while adjectives like "dim," which describe something as emitting very little light, are used to describe someone as having low intelligence, someone like a halfwit.
An informal or slang meaning of the word "juice" is power, especially electricity, even more especially that from a battery. If a battery works, it has "juice." If it doesn't, it's "dead." With that in mind, if you have a light or light bulb that is electrically powered by a battery but that battery has very little "juice," that light is dim, and the less "juice," the dimmer it becomes until it doesn't shine any light at all.
Based on that, the speaker is saying that the so-called "halfwit" has quite uncharacteristically said or done something relatively smart, something that is not stupid. The speaker is implying that the so-called "halfwit" is so dim he thought he maybe had no "juice" (i.e., so little battery power that his light looked completely off), so with "still," he's expressing his surprise that his batteries weren't dead, that he had the "juice" to not be dim but maybe even be a little bright, even if only for a moment by doing or saying something bright or brilliant.
Basically, it's a heavily backhanded compliment (i.e., an insult disguised as a compliment but intentionally disguised so badly it's not really disguised at all). The speaker is "complimenting" the person for saying or doing something relatively smart while at the same time insulting that person by saying he's normally so stupid that one can't tell his brain even works, that his wit (brain) is so dim anyone would think its batteries were dead (had no "juice"), hence the speaker being surprised that the halfwit, or dimbulb, has "still got some juice."