To me a libertarian is just anyone who thinks everyone is, and should be, a sociopath, and build their views off of that. Although they prefer the term "self-interested rational actor". Or they might not even admit that terminology, but if you dig into their views they will reduce to it even if they don't realise that themselves.

Of course it's well known by now, and even most modern economists have accepted this: the vast majority of people are not in fact wholly self interested. Though they are still rational in the sense that preferences are transitive.

Is there a political philosophy less understood, or more mischaracterized, than Libertarianism?

By its own adherents? Probably not.

Maybe, but the people of influence who profess to be libertarians are not exactly committed adherents of a carefully thought-out libertarian philosophy (except maybe Thiel, although his libertarianism has a religious caste). In general, the SV libertarian is an egotist who talks up a poorly understood distortion of libertarian thought because it's a fig leaf for their rapacious amorality, and they cast off its tenets whenever it suits them.

Probably not since even among themselves they can’t agree what it really is.

One thing universally agreed is that everyone should stay away of them at parties.

Between libertarians and Lisp people.. I don’t know man.. :)

In general most people fit the widest definition of Libertarianism which is:

"People should be free to do whatever they want except for these specific conditions which I consider important and do not negatively impact me very much but may negatively impact others a lot"

Sure, Marxism.

Nothing is less understood, or more mischaracterized, as Anarchism (Libertarian Socialism).

(And, no, Libertarianism is not Anarchism. It is Libertarian Capitalism.)

Isn’t kind of the point of Anarchism that everyone is free to make up their own mind on what Anarchism means?

What? It is a word. Words have meanings.

Anarchism is a political philosophy and movement that is skeptical of all justifications for authority and seeks to abolish the institutions it claims maintain unnecessary coercion and hierarchy, typically including, though not necessarily limited to, governments, nation states,[1] and capitalism. Anarchism advocates for the replacement of the state with stateless societies or other forms of free associations. As a historically left-wing movement, usually placed on the farthest left of the political spectrum, it is usually described alongside communalism and libertarian Marxism as the libertarian wing (libertarian socialism) of the socialist movement.

Yeah, methinks the joke slipped past… I know what anarchists tend to think anarchism means… what’s funny to me is that “words have meanings” — those s’s are significant there — implies either an appeal to an unspecified authority that an anarchist should be skeptical of, claim to being such an authority that any other anarchist should also be skeptical of, or deference to some vague majoritarian tyranny that any anarchist should reject out of hand.

Libertarianism of any stripe has this same bootstrap problem: if I’m not free to redefine words, then I’m not free.