In several different team sportsball variants this is a far more important concept than onside in that a penalty shall be assessed if observed by a referee.
Stemming from the contraction of off the strength of his side, offside began being used in the 1860s to indicate when a player is not where they are supposed to be based on the rules of the game being played. Offside rules exist not only as a means of restricting player positioning to encourage more strategic team play but also as a safety measure against severe injury. Such rules are defined and administered differently across the world of sportsball, as well as historically within a particular variant. This illustrates that being offside is a bigger no-no in some sportsballs than in others, and in some years more so than in others. Some don't even care and have no rules or regulations against it, either now or then.
SQ '24