What’s the origin of the use of the word “hard” to mean unequivocal?

Is it really slang? I think this is the same sense:

If they sell any [corne] at home, beside harder measure, it shal be dearer to the poore man by two pence or a groate in a bushell then they maye sell it in the market.

From 1577.

I found this in the OED under the sense that also has “hard deal/bargain”. It says that “In early use with implication of short measure”.

“Hard bargain” itself is a pretty old term:

Christ doth make an hard bargaine with none.

From a1628.

_{“Short measure” falls under this OED definition of “short”: “Not coming up to some standard of measure or amount; inadequate in quantity. short measure, short weight: defective quantity by measure or weight; also, a measuring rod, vessel, etc., or a scale-weight, which defrauds the purchaser.”}

Hard does not mean "unequivocal" - Hard is a close synonym of both "solid" and "firm", i.e. by extension = resistant.

OED:

A. adj. I. Not soft; resistant to force or pressure; firm, solid, unyielding; robust.

2b. Firm, steadfast; unyielding. Chiefly of a person, with respect to belief, resolve, etc.

The meaning of "hard" in "a hard "No"." has not changed in over 1,000 years: 3.a.

3 a. Not easily moved or affected emotionally; resistant to pity or entreaty; unfeeling, callous. Also in hard of heart. Cf. hard-hearted adj. See also to die hard at die v.1 3b. OE Blickling Homilies 57 Manige men beoð heardre heortan.