Does Webster get to answer?
Not if the question is asked in the Catholic context of Lent.
Since abstinence from meat during some days in Lent is a Catholic law, the definition of meat must come from the same lawmaker enjoining the abstinence from meat - that is, the Church. Not Webster. Nor you, nor the National Chefs Association. When it comes to abstinence in the Catholic Church, "meat" is what the Catholic Church defines it to be.
How does the Church define "meat?"
For thousands of years, in law and in practice, the Church has defined meat as the flesh, marrow and blood of animals and birds as constitute fleshmeat (viand). As proof that the Church's definition of meat included fowl, remember that, many years ago, even the products of such animals - cheese, butter and eggs (aha!) - were also prohibited during certains days of Lent.
If one couldn't eat the eggs of chickens, then how could the Church say the chicken itself could be eaten?
Furthermore, at one time, even on non-fast days in Lent, one could not mix meat and fish in the same meal. In Catholic mentality, there are only two categories : meat and fish. Chicken is not a fish; it is a meat.
The 1910 Catholic Encyclopedia says that the definition of meat comes from what intelligent people normally consider meat. Ask yourself this, if you wanted to buy chicken one day, went into the supermarket and saw only two signs - left for meat, right for fish - would you turn left or right, to find the chicken?
Even Webster isn't helpful
Even if one wanted to get all definitive answers from Webster (who made that dictionary the ultimate law?), you get different answers. Why? Because dictionaries don't mandate meanings; dictionaries describe meanings. Word meanings are derived from people's usage. So, every now and then, there are new editions published of each major dictionary as words die, words are born and words change.
So good old Webster has multiple definitions for meat, because people use that one word with a variety of meanings :
1. As the edible part of something, as opposed to its husk or shell. So, there's even something we call coconut meat, though it isn't prohibited in Lent by the Church, because that's not what the Church means by "meat."
2. As solid food, as opposed to drink.
3. As animal tissue, used as food. Thus we can even say that a certain fish is very meaty, as opposed to another kind of fish which is less meaty.
4. The flesh of mammals, as opposed to fowls or fish. But this isn't what the Church means when it speaks of meat.
5. The flesh of domesticated animals, as opposed to wild ones, like jungle boars or tigers. Yet everyone would agree that a wild boar pork chop is meat, or that tiger steak is meat.
Let's Face It
The doubt about chicken as meat never came up till the 1970s. Let's be honest. Changing definitions from what was once understood and unquestioned concerning poultry is just our way of getting out of giving up all meat on abstinence days in Lent.