On the course side is an expression used in

sailboat racing The sport of sailing involves a variety of competitive sailing formats that are sanctioned through various sailing federations and yacht clubs. Racing disciplines include matches within a fleet of sailing craft, between a pair thereof or among ...

to indicate that a boat was on the wrong side of the starting line when the starting signal was given. According to the Sailing Instructions valid for a specific racing event, being on the course side may entail an immediate disqualification, or it may allow to maneuver for correcting the start procedure. The corresponding scoring abbreviation is OCS, whose technical meaning is as follows:http://www.sailing.org/tools/documents/WorldSailingRRS20172020new-%5B24067%5D.pdf

References

Sailing (sport) {{sailing-stub