When a noun has more than one adjective some time I see comma is used between them. Is it just an incorrect use of comma? e.g "Unwind by a nice hot bath" vs "Unwind by a nice, hot bath".
Also "There is a big, flat, windy beach." vs "There is a big flat windy beach."
Certain adjectives also have a potential role as intensifiers of other adjectives that can verge on the adverbial. This applies both to some that have positive connotations, such as nice, good, bright; and to some with less positive connotations, such as silly, fat, crappy.
When they are used in this way, they are not separated by commas from the adjectives they pertain to. They may still be tinged by their ostensible meaning (as in the case of your "nice hot bath" example), but usually the specific sense that is normally carried by the adjective recedes in favour of a generalized aura of positivity or negativity.
Compare, for instance
He seems like a nice young man. [A generalized affirmation of likeability.]
with
He seems like a nice, young man. [A slightly more nuanced and specific appraisal of his apparent good or desirable qualities.]
Similarly:
Why does anybody even pay attention to that stupid fat cow? [A generalized put-down or formulaic rejection.]
Why does anybody even pay attention to that stupid, fat cow? [A possibly more nuanced and specific dismissal of her apparent deficiences.]
I can't think of many instances where I have seen more than one such intensifying adjective juxtaposed with a regular adjective; in other words, they are not strung together in lengthy trains.
Write.com, while not advocating confusing or cavalier usage of the comma, essentially says that comma usage is governed by fewer rules than many people have been taught:
Pauses
Any comma essentially represents a pause – one that gives your readers the signal to take a quick breath before continuing with the next portion of the sentence. When a comma is used, it places more emphasis on what immediately precedes and follows the comma. Sometimes, there is no grammatical rule that dictates the use of a comma in a particular spot within a sentence, but the use of one, creating a pause, can create an emphasis that does not exist without the comma. Consider the following examples:
Example 1: Yesterday, I experienced the worst day of my life. Example 2: The novice guitarist was butchering my favorite song, quite badly.
In both examples, the comma is grammatically unnecessary. It does, however, create a pause that places more emphasis.
So, do you want "There is a big flat windy beach" or "There is a big, flat, windy beach" to be read?
Note that this does not deny '*grammatical' (ie 'to differentiate types of construction, to make for easier reading ...') usages, of which there are many. Erik's answer shows one pointing to a subtle change in meaning.
A single comma between two adjectives, as in your bath example, tends to parse the sentence wrongly: in this case, "unwind with a nice" and "hot bath". ("Unwind by" suggests, say, doing yoga on the bathroom floor while you wait for your bath to cool.) Commas in pairs are OK, but better omitted unless you want to slow the reader to make the separate importance of each one sink in: "a nice hot deep bath" is fine; "a nice hot, deep, bath" puts the emphasis on depth; while "a nice, hot, deep bath" stresses perhaps the importance of heat; "a nice, hot, deep, bath" tells of a nice bath that is also hot and deep. Arguably these distinctions are hair-splitting. The simple rule is to omit commas between adjectives unless you have a specific reason to do otherwise.
