Slang term used by British young people in the late 20th/early 21st century. It seems to be used more in a school/education setting than anywhere else. Examples of usage:
"That test was such a doss!" : The examination that we have just undertaken was extremely easy.
"I'm gonna doss off for a bit." : Please excuse me while I sit myself down and do nothing.
"What a doss homework!" : That was a surprisingly easy homework. I did not have to engage my brain at all.