Decorum

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mid 16th century (as a literary term, denoting suitability of style): from Latin, neuter of the adjective decorus ‘seemly’.

wiktionary

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From Latin decōrum, neuter form of decōrus(“proper, decent”).

etymonline

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decorum (n.)

1560s, "that which is proper or fitting in a literary or artistic composition;" 1580s, "propriety of speech, behavior, or dress; formal politeness," from Latin decorum "that which is seemly," noun use of neuter of adjective decorus "fit, proper," from decor"beauty, elegance, charm, grace, ornament," from decus (genitive decoris) "an ornament," from PIE root *dek- "to take, accept" (on the notion of "to add grace").