grammar
Grammar is the hidden machinery of a language — the whole system of rules that lets a string of sounds or words add up to meaning. It tells us how to build words, how to slot them together, and which combinations land as natural and which as nonsense. You carry this machinery in your head: "the cat sat on the mat" feels right, "mat the on sat cat the" feels broken, and no one had to teach you why. That instant sense of right and wrong is grammar at work.
Here's the twist most people miss. There are two very different things both called grammar. Descriptive grammar is what linguists do: they watch how people actually speak and write down the real, often beautifully intricate rules behind it — including the rules of casual speech and every dialect. Prescriptive grammar is the list of dos and don'ts from style guides and schoolteachers: don't split infinitives, never end a sentence with a preposition. The first describes; the second judges.
So when someone says a way of speaking has "no grammar" or is "just lazy," they've mixed the two up. Every variety of every language — from formal writing to street slang to a signed language — runs on a rich, consistent grammar. What people usually mean is that it breaks the prescriptive rules of a prestige standard, which is a social preference, not a fact about whether the language has rules. It always does.
"Who did you give it to?" vs. "To whom did you give it?" — both are grammatical; only one obeys a prescriptive rule.
Descriptive vs. prescriptive: everyday speech follows its own consistent rules, even when it breaks a style-guide preference.
"Grammar" comes from the Greek grammatikē technē, "the art of letters." Oddly, so does "glamour": in the Middle Ages "grammar" (learned Latin letters) also carried a sense of arcane learning and even magic ("gramarye"), and in 18th-century Scots an altered form, "glamour," came to mean an enchanting spell.