What does 鴨が葱を背負って来る mean?


I checked [the meaning of Farfetch'd on
Bulbapedia](https://bulbapedia.bulbagarden.net/wiki/Farfetch%27d_\(Pok%C3%A9mon\)#Name_origin)
and it says it is "カモネギ", likely to be inspired by "鴨が葱を背負って来る", meaning
"something surprising, but convenient".

What does "surprising but convenient" mean? Is it like an idiom or phrase that
is often used? Can some examples be given how it might be used in daily
speeches in a situation or context?

For example, does it mean good or bad? Can it be: I didn't think it'd rain,
but it suddenly did and made my car clean; it really is 鴨が葱を背負って来る.

I saw [an example of
「あんな高額な商品を買ってくれた上に、家族や友達にまで勧めてくれるなんて、鴨が葱を背負ってくるとはこのことだ」](http://kotowaza-
allguide.com/ka/kamoganegisyotte.html), but I don't quite get what it means.

I also saw [some website explains it as "Along comes a sucker just begging to
be parted from his money"](https://en.bab.la/dictionary/japanese-
english/%E9%B4%A8%E3%81%8C%E3%81%AD%E3%81%8E%E3%82%92%E8%83%8C%E8%B2%A0%E3%81%A3%E3%81%A6%E6%9D%A5%E3%82%8B).




Basically it means "easy hunt/game/prey". I think "something surprising but
convenient" is slightly wrong. So it can't be used like your example. Second
example is correct, the phrase exists for.

We love 蕎麦(Japanese noodle), and duck(鴨) meat one is really popular since Edo
period. We usually put 葱(Green leek?) in 蕎麦, so if we found a 鴨 carrying 葱 and
could hunt it, we can get all ingredients for 鴨南蛮(duck meat Soba). It's how
this phrase was born. I know it's impossible. haha.

Also, this phrase is basically used for bad meaning, sneering someone.

