Late 17th century: from modern Latin trajectoria (feminine), from Latin traject- 'thrown across', from the verb traicere, from trans- 'across' + jacere 'to throw'.

• jet from late 16th century:

  The name jet for a hard black semi-precious mineral comes ultimately from the Greek word gagatēs ‘from Gagai’, a town in Asia Minor. When we refer to a jet of water or gas, or a jet aircraft, we are using a quite different word. It comes from a late 16th-century verb meaning ‘to jut out’, from French jeter ‘to throw’, which goes back to the Latin jacere ‘to throw’. Jut (mid 16th century) is a variant of jet in this sense. Jacere is found in a large number of English words including abject (Late Middle English) literally ‘thrown away’; conjecture (Late Middle English) ‘throw together’; deject (Late Middle English) ‘thrown down’; ejaculate (late 16th century) from jaculum ‘dart, something thrown’; eject (Late Middle English) ‘throw out’; inject (late 16th century) ‘throw in’; jetty (Late Middle English) something thrown out into the water; project (Late Middle English) ‘throw forth’; subject (Middle English) ‘thrown under’; trajectory (late 17th century) ‘something thrown across’. Especially if you use budget airlines, air travel today is far from glamorous, but in the 1950s the idea of flying abroad by jet aircraft was new and sophisticated. At the start of that decade people who flew for pleasure came to be known as the jet set.