1 unsuitable or not correct for a particular use or occasion: --
Is it considered improper to wear such a short skirt to a formal occasion?
improper prescription of medicines
2 related to sex in a way that is rude or socially unacceptable: --
I trust you're not making improper suggestions to my husband!
4 not suitable or correct for a particular use or situation: --
an improper choice of words
5 dishonest, and not allowed by a rule or by law: --
improper accounting/payments/trading
The cropland may have been polluted because of improper use of pesticides and fertilizers.