It means thank you, a polite rejection, but I am embarrassed to say it, so I use gifts instead of words. This is my own understanding.

It means that you are obviously not suitable. , because crabs are reptiles. To put it bluntly, it means to tell you to crawl

Giving crabs means that you are out of the way. This confession is like feeding a dog. Crabs walk sideways, which means to let you get out of the way. To put it bluntly, it means that we are not in a straight line