Howseitgoin words are dependent on social usage.
Not entirely - fortunately, because otherwise communication would be a very hit-and-miss affair!
Sometimes it is essential to have an agreed definition of words, and the law is one area where definitions can't be left to the vagaries of social usage -
robbery? breaking and entering? burglary? They all sound pretty much the same, they are all about nicking stuff, but they have precise legal definitions.
The UK Supreme Court ruled that a precise definition of 'man' and 'woman' exists, i.e. the established, scientific, biological definition of human sex as binary and immutable.
It referred directly to the Equality Act, but logic would suggest that wherever the words 'man' or 'woman' appear in legislation, they follow the biological definition, rather than social usage based on a recent, irrational, passing fad that made them interchangeable.