1. indicates that something is not suitable, appropriate, or decent; violates social norms or standards
It is improper to speak with your mouth full.
É impróprio falar com a boca cheia.
2. indicates that something is not correct or accurate in a technical or formal sense
It is improper to use informal language in an academic essay.
É impróprio usar linguagem informal em um ensaio acadêmico.