general

1250–1300; Middle English < Latin generālis, equivalent to gener- (stem of genus) genus + -ālis -al¹

1, 2. General, common, popular, universal agree in the idea of being nonexclusive and widespread. General means belonging to, or prevailing throughout, a whole class or body collectively, irrespective of individuals: a general belief. Common means shared by all, and belonging to one as much as another: a common interest; common fund; but use of this sense is frequently avoided because of ambiguity of sense. Popular means belonging to, adapted for, or favored by the people or the public generally, rather than by a particular (especially a superior) class: the popular conception; a popular candidate. Universal means found everywhere, and with no exceptions: a universal longing.

English general (adjective and noun) comes via Old French from the Latin adjective generālis “common, belonging to all, belonging to a race or genus” (as opposed to speciālis “particular, belong to a species”).
The military sense of general (noun) dates from the mid-16th century; general officer and general of the army date from the mid-17th century. The heads of some Roman Catholic religious orders are called generals, a usage that began in the 16th century and applied originally to the Jesuits but now applies to the Franciscans and Dominicans as well; the usage was extended in the 19th century to the head of the Salvation Army.
The phrase in general originally meant “in a body, collectively” (a sense that is now obsolete), with a clear relation to the sense of the adjective general “with respect to a whole class, group, or category.” It later developed further to mean “usually,” which can be interpreted as meaning “as a general rule (rather than a specific instance).”