I saw the word obtrusive the other day and I figured it was a good word to etymologize.
Obtrusive first showed up in the mid seventeenth century from the classical Latin obstrus-, but obtrude first showed up a century earlier. Its Latin origin is the word obtrudere, thrust into, press upon, or even presume. The ob- means in front of or toward, while trudere is thrust or shove. I guess something that’s obtrusive is shoved in your way. Trudere is also from the Proto Indo European treud-, which means press, push, or squeeze and is the origin word for threat. Which actually sounds like it has a more interesting origin than obtrude.
Intrusive has a kind of different, kind of similar origin. It showed up earlier, in the fifteenth century, from intrus-. Not much different so far. Intrude showed up in the early fifteenth century as a church word that meant to take possession of part of a church that doesn’t belong to you. Wow, specific. It wasn’t until the mid sixteenth century that it started to mean what we use it as. In any case, it’s either from intrusion (which showed up in the late fourteenth century) or directly from the classical Latin intrudere, to thrust. The in- means in (shocking!) and the trudere is the thrust, so thrust in. Which is pretty intrusive. Plus there’s also extrude, which means to thrust out, the ex- of course, being the out part.
Finally today, we’re looking at protrude. It showed up in the early seventeenth century (and protrusion not until the middle of the century), initially meaning thrust forward before it meant something that sticks out. It’s from the classical Latin protrudere, which means protrude or push, the pro- meaning forward and the trudere… well, you know. Thrust forward. I guess something that’s protruding is being pushed (or thrust) out?