The word "diagonal" to represent "/", or "forward slash" in transmitting wind information from aircraft has been in popular use since I was a kid, and that's going back a very, very long time! It probably originates from the old days of poor quality HF R/T, where you can still hear it being used even though modern SSB is almost as good as a phone call. As TheOddOne points out, it isn't used by ATC.
For TheOddOne's info... there are many words which have different meanings around the world. In my pastime of birding the word "Jizz" is a word used in the UK to describe bird behaviour.. but try using it across the pond.