Gymnasium
late 16th century: via Latin from Greek gumnasion, from gumnazein ‘exercise naked’, from gumnos ‘naked’.
wiktionary
From Latin gymnasium, from Ancient Greek γυμνάσιον(gumnásion, “exercise, school”), from γυμνός(gumnós, “naked”), because Greek athletes trained naked.
etymonline
gymnasium (n.)
1590s, "place of exercise," from Latin gymnasium "school for gymnastics," from Greek gymnasion "public place where athletic exercises are practiced; gymnastics school," in plural, "bodily exercises," from gymnazein "to exercise or train," literally or figuratively, literally "to train naked," from gymnos "naked," from a metathesis of PIE *nogw-mo-, suffixed form of *nogw- "naked" (see naked).
A feature of all ancient Greek communities, at first it was merely an open space, later with extensive facilities and including training for the mind as well as the body. Hence its use in German from 15c. as a name for "high school" (more or less paralleling a sense also in Latin); in English it has remained purely athletic. For the "continental high school sense," English in 19c. sometimes used gymnastical as an adjective, gymnasiast for a student.
