1 unacceptable according to the normal standards of moral, social, or professional behaviour: It is quite improper for you to have an affair with one of your students. | an improper suggestion (=about sex)
2 illegal or dishonest: allegations of improper banking practices
3 not correct according to a set of rules: the improper use of a singular verb with a plural subject
— improperly adverb: If you are improperly dressed, you will not be admitted.

Longman dictionary of contemporary English. 2004.