on

on (prep.) O.E. on, unstressed variant of an "in, on, into," from P.Gmc. *ana "on" (Cf. Du. aan, Ger. an, Goth. ana "on, upon"), from PIE root *an- "on" (Cf. Avestan ana "on," Gk. ana "on, upon," L. an-, O.C.S. na, Lith. nuo "down from"). Also used in Old English in many places where we would now use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877. On time is from 1890.