For example, in “hard no” or “hard pass”
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.
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.
“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.
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.
In the card game blackjack, the ace can have the value 1 or 11, at the player's discretion.
A hand consisting of an ace and a seven card would be a "soft 18," meaning it could be counted as 18 points (if the ace were treated as 11), or as 8 (if the ace were treated as 1). You might consider taking another card to get closer to 21 without exceeding it (the objective of blackjack), because you could count the ace as 1 if the new card were greater than three.
A hand consisting of a ten card and an eight card would be a "hard 18." Taking an additional card with such a hand would be risky.
So, a hard hand in Blackjack is much less flexible than a soft hand.