someone that causes an entire room of people to miss out on a funny joke, usually out of either shyness or selfishness, by solely mouthing the gem in a whisper to someone next to them. Thankfully, the "joke poacher" completely solves this tragic dilemma in a swift act of altruism by restating it audibly.
Post-laughter, it is not uncommon for middlemen like the "joke poacher" to become the target of animosity from the originator, even if said poacher gives them credit.
(Setting: Large family shopping for Rx glasses)
Femme 1: (whisper) "The best way to tell if the glasses are the wrong prescription is if Mike's head appears normal"
Joke poacher: (audibly and/or loud): "The best way to tell if the glasses are the wrong prescription is if Mike's head appears normal"
Everyone in room: AHAHAHAHAHA
Femme 1: "wtf poacher, that joke belongs to me" (tears)
Joke Poacher: "No one would have ever heard it genius, you are such a chuckle-choker"