This takes place accidentally when a person attempts to be smart and witty but fails completely, leaving an awkward silence.
This can be used with an out dated pun, word, or joke; the only big requirement is that the person on the receiving end be confused and silenced by the odd response.
This word comes from the word comeback with the "back" meaning retort. But in the word comewith, the "with" implies that the person attempting to use the retort is just making them look like an even bigger idiot.
This can also happen when someone rambles on and on, or when they repeat exactly what someone has just said.
Alice: Alright, before I sign up for the trip to Spain, I need to know what I'm paying for and how much I'm paying for it. Like plane tickets, ...
Marissa: Yeah, we need to know the final price of round trip plane tickets...
Alice: Hotel room costs, ...
Marissa: Hotel rooms...
Alice: Bus fares, ...
Marissa: Bus fares...
Alice: Why are you repeating exactly what I'm saying? It's not funny. I'm not trying to be funny, and you repeating it isn't making it funny either. It's almost like your idea of being funny is just using a bunch of comewiths.
Mary: Ew, the school lunch today is nasty.
Kelly: What? Really? I like it!
Mary + Amy: *stare*
Kelly: Hahaha! Psych!
Amy: Ugh... Did you really just say 'psych'?
Kelly: Um, yeah?
Mary + Amy: *complete and utter silence*
Kelly: That wasn't a comewith just now, was it?
Mary: Unfortunately, it was. And it was pretty bad, too.