What cyclists say by convention as they pass each other on the road, or occasionally as they ride near pedestrians. It is infinitely more cool than mounting a dorky little horn or bell on your handlebars and squeezing it at every opportunity. It may not be always be strictly necessary, but it is a little courtesy that will get you cred and make you look experienced.
"On your left! On your leeeeeft!" You gotta let us know! Where are your manners?"
The phrase "shag your left toe" originates from modern day england. This phrase is normally used by fuckwits who think they can stick their dick in the most irrelevant parts of your body.
This Phrase has no meening, as it is just and irrelvant comment.