On Taiwanese Facebook page I read a conversation under a post from a hospital. A lot of people of course write 保重 (get well), and the poster answers 甘蝦 to all of them. When looking up this in a dictionary all I can understand is that it means "sweet shrimp" ...
So what does 甘蝦 mean?
This is because Taiwanese Hokkien doesn't have its own official written form. It has a romanization, but none of the Taiwanese know it, despite most speaking the language fluently.
So when they want to express something in Taiwanese while writing, they would either use ZhuYin 注音, or for the more common words, Chinese characters that sound similar in Mandarin, chosen randomly but turned into a kind of standard slang.
甘蝦, 多蝦 are common (感謝, 多謝), there is even a song by Mayday, "金多蝦" (真多謝).
Other examples include 拍寫/拍謝 (歹勢) "sorry, excuse me".
三小,沙小 is a very rude way of saying "what", a bit like wtf (啥潲).
Some more examples here.
It's just another internet slang phenomenon, not only Taiwanese words get this. 三Q = thank you, common to both Chinese and Japanese, or romanizing random words hen=很 (you'll see things like 我hen開心) or der=得, sometimes derrrr.
It is a similar pronunciation of Hokkien 感謝/感谢, so the meaning is to thank.
The 臺灣閩南語常用詞辭典 shows that the pronunciation is kám-siā.
It is a style of humor, not a joke. We use them a lot.
Another example is 蝦米/虾米. It is the similar pronunciation of Hokkien 什麼/什么 (what). This one has been spread to China already.
sia sounds like 蝦, and it is easy to type.