write (up)on someone or something
1. . to write about someone or something. (Upon is very formal here and much less commonly used than on.) I had to write an essay, so I wrote on my uncle. What are you going to write upon?
2. . to write on someone's skin or some thing. (Upon is formal and much less commonly used than on.) Don't write on Billy. After all, he's your brother. Who wrote on this page of the book?
3. . to use someone [such as some one's back] or something as a flat base to support something that is being written upon. I have to sign this check. Here, let me write on you. Do you mind if I write on your desk? I just need to sign this. 771 writer's block