For everyone who don't know, this is the complicated version of Chinese numbers. In modern days, they are mostly used in writing cheques, because these characters are not as easily modified as the simple version.
How do they decide what the complicated character for 7 should be? Why does it include the symbol of a tree? Can natives derive the meaning of a new symbol by its components or are they just as clueless as we are until they learn the word?
The character used to refer to a type of tree sap, but shares the same pronounciation as 七. I'm guessing the reason it's considered offensive is because the top component implies ejaculation, but that's something I'd need to check!