"Hard" vs. "hardly"

Etymonline has this:

hardly c.1200, "in a hard manner, with energy and force," from hard (q.v.). Main modern sense of "barely, just" (1550s) reverses this, via the intermediate meaning "not easily, with trouble." Formerly with superficial negative (not hardly).

Wiktionary definition of Hardly has four meanings. Out of the four, only one is in contemporary use, or so it seems(Hardly, as in barely, scarcely etc..)

hardly (comparative more hardly, superlative most hardly)

1. (manner, obsolete) Firmly, vigorously, with strength or exertion.
2. (manner, archaic) Harshly, severely. I can't really deal hardly with people.
3. (manner, obsolete) With difficulty. He made his way hardly through the enemies to the castle.
4. (degree) Barely, only just, almost not. I hardly ever watch television. I hardly think they'll come in this bad weather. It is hardly possible he could lose the election.

Etymologically, the meanings makes sense with all the first three. But modern usage tends to favour the fourth meaning more

"Hard" means difficult. No confusion there.

"Hardly means "difficult to COME BY." That is something is BARELY there, or "conspicuous by its (near) absence." That appears to be the etymology. It is NOT an adverb of "hard," which is why it is confusing.