I watch many videos of Hikaru Nakamura and I hear very often the term "juicer." Is it a slang word to refer to the bishop?
Does “juicer” mean bishop?
This YouTube video of Hikaru playing the Beth Harmon bot gives some clues.
First, at 3:06 he says "I think I can just throw in a juicer check" and indicates a bishop check with arrows on the board. This suggests that juicer does mean bishop for him.
However at 3:33 he says "I don't understand. I'm just up a juicer, aren't I?" when he is up a knight.
Then at 3:50 he says "Let's block the juicer, no nonsense." as he blocks a pawn.
Finally at 7:00 he says "A juicer is just whatever you want it to be. The juice is the juice"
Hikaru Nakamura uses the term "juicer" to denote an opponent's piece, generally ascribing to the piece some strategic value. Nakamura uses the term roughly as a substitute for "piece", referring to any part of the opponent's army including pawns (maybe excluding kings, since they cannot be captured?). He would not use the term "juicer" to describe other types of strategic boons such as good positions or time advantages.
Nakamura tends to use it to describe bishops, but as noted in the question, this is not a strict rule. Nakamura has a host of other nonstandard Hikaru-isms as well, for example calling knights "horses".
This slang likely comes from the idea of something being "juicy", meaning rich or abundant in contents. Thus, a knight or bishop could be a juicy target for Nakamura when inadequately defended.
It is worth noting that the term is also big on Twitch, and Nakamura is one of the largest chess streamers on that platform. Another Twitch streamer who uses the term "juicer" frequently is xQcOW, although his usages are not only reserved for chess pieces and can be just about any object of any perceived value. It is also of note that although xQc is a variety streamer known for being a former professional Overwatch player, he has also been involved in the amateur chess scene recently, and without having researched it I do not know which of the two streamers devised the usage (or whether it was originally picked up from another source or not).