dec

The root dec comes from a small family of related Latin words built on one idea: fitting in. The verb decēre meant "to be fitting, to be becoming" — to suit a person or an occasion. The Romans used it the way we say "that looks good on you": a certain robe decēre a senator; a certain restraint decēre a guest at a feast. From the same idea came two nouns, decus and decor, meaning "what becomes a person" — and that came to mean both dignity / honor and beauty / grace. The Latin instinct was that what is proper is also pleasing to look at: the right thing and the good-looking thing are the same thing.

From this single seed, English grew two branches that at first seem unrelated.

The first branch is about behaving properly. If something suits the occasion, it is decent — literally "fitting." That is why decent slides so easily between "morally proper" (a decent person), "socially acceptable" (decent clothes), and "good enough" (a decent meal): all three are versions of suitable for the situation. Add -cy and you get decency (the quality of being fitting); negate it and you get indecent / indecency (not fitting — improper, obscene). The Latin noun decōrum kept the formal sense directly: decorum is "the fitting thing" raised to a code of behavior — proper conduct, etiquette. From the adjective decōrus English took decorous (behaving in a fitting, dignified way).

The second branch is about making things beautiful. The verb decorāre meant "to make fitting / to make handsome," i.e. to adorn. From it: decorate (to add beauty to), decoration (the ornament, or the medal of honor — a fitting reward), decorative (serving to beautify), and the French-flavored décor / decor (the overall fitting-out of a room). Re- gives redecorate (decorate again).

So the whole family unfolds from one Latin reflex — what fits is what is good and beautiful: "fitting behavior" → decent, decency, decorum, decorous; "fitting appearance" → decorate, decoration, decor. One caution: decade and decimate look like they belong here, but they don't — they come from decem ("ten"). A decade is ten years; to decimate originally meant to kill every tenth man. Same letters, completely different root.