A set of rules used for paintings such as in the 18th century in England (e.g. by Thomas Gainsborough). His paintings reflect a stiff position, aura of seriousness and dignity which must be shown on the portraits
"Dequorum" is not a recognized term or word in the English language. It does not have a widely understood meaning or definition. It is possible that it may be a misspelling or a word specific to a certain context or field.