What is the origin of the phrase "hard and fast rule?"

I just used this phrase in answering another question, only to realize that

1. I didn't know its origin and
2. it is usually used in the negative, as in "..it's not a hard and fast rule, but..."

I'd like to know the origin, and perhaps why it seems to be more commonly used in the negative.

Here, fast doesn't mean "(capable of) moving quickly". Much rather, it is being used in the sense "firmly fixed" (see fasten your seatbelts or fast friends). The Phrase Finder says that "This is a nautical term. A ship that was hard and fast was simply one that was firmly beached on land." It adds that the term was used in figurative sense by the early 19th century.

Personally, I don't think it's being used more commonly in the negative. Indeed, a quick COCA search returns 40 occurences of "hard and fast rule" or "hard-and-fast rule", but only 22 of them are using it in a negative context — and I am being as generous as possible there, counting not only "no hard and fast rule" and "not a hard-and-fast rule", but also "don't have any hard and fast rule", "rather than any hard and fast rule", "was never a hard-and-fast rule" and the like.

One thing stands out to me, though: out of 16 occurrences of "hard-and-fast rule", with hyphens, 12 appear in a negative context, or 75%. For the non-hyphenated version, it's almost the other way round: 60% positive, 40% negative. (Again, counting "negative" very generously.)

The figures from the BNC are too small to be statistically meaningful. But anyway, here's an overview:

                              COCA                   BNC
                        total   negative      total   negative
hard and fast rule        24       10            8        7
hard-and-fast rule        16       12            1        1 

I agree that it tends to be used in the negative, and I think the reason is pragmatic.

If you are saying there is a rule for something, you can in principle point to the rule: you don't need to qualify it. (You might say there is a "specific rule" or a "definite rule". You might say that there is a rule but it's not certain whether or not it applies to this case. But there is not any pragmatic need to qualify the rule).

If you are denying the rule, it is much harder to be precise: we often don't know all the rules, or don't know exactly what they say; so there is attraction in being a little bit vaguer. "There is no rule against ..." is a precise statement, which might easily be shown to be wrong, whereas "There are no hard and fast rules against ..." is comfortably vague.