I don't say "On your left" any longer - I say "Did that hurt?"
All seriousness aside, I speak in complete sentences to MUP-dwellers -- I say "I'm a cyclist passing you on your left" That seems to work. But passing a pedestrian on a single speed Schwinn Meridian upright Trike (the "Schwinster") usually takes upwards of a half-hour.
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Trikeman