"minor non-speaking role," 1902, theatrical slang, from the verbal phrase walk on, attested in theater jargon by 1897 with a sense "appear in crowd scenes," from walk (v.) + on (adv.). Meaning "actor who has such a part" is attested from 1946. The sports team sense is recorded from 1974.

also onsite, 1959, from on + site. Originally in reference to Cold War military inspections.

"ruse, deception," 1937, from earlier adjectival meaning "assumed, feigned" (1620s), a figurative extension of the verbal phrase on the notion of putting on costumes or disguises. To put on (v.), of clothes, garments, etc., is by early 15c.; see from put (v.) + on (adv.). Hence "clothe, cover, assume as covering" (mid-15c.) and "assume the garb or appearance of" (real or feigned), 1520s. The expression put (someone) on "play a trick on, deceive" (by 1958) seems to be a back-formation from the noun.

1959, from the verbal phrase, from iron (v.) + on (adv.).

that which arouses or excites, 1968, originally of psychedelic drugs, from verbal phrase turn on "activate (a mechanism)" (1833), specifically from figurative sense turn (someone) on "excite, stimulate, arouse," recorded from 1903; from turn (v.) + on (adv.).

by 1969, originally in reference to the use of computers in education; see hand (n.) + on (adv.).

"intermittently, occasionally," 1530s; see off (adv.) + on. As an adjective, "occasional," from 1580s.

1909, "with one side facing," from side (n.) + on (adv.), perhaps based on earlier head-on. In reference to a vehicle collision, by 1828.

"concerning, regarding," late 14c., from touch (v.), on model of French touchant.

"gentle rubbing with the palm of the hand," 1886, from French effleurage, from effleurer "to graze, touch lightly, touch upon, strip the leaves off," from ef- "out" (see ex-) + fleur as in the phrase à fleur de "on a level with," from German Flur "a plain, field, meadow" (see floor (n.)).