lapsus linguae – a slip of the tongue (used when a speaker says something, often insulting, and then claims that it was not a genuinely held belief).
sine qua non – very important person (used to show that a person is indispensable).
persona non grata – unwelcome person (used to show that a person is unwelcome)
Qui tacet consentire videtur – He who is silent seems to agree (used to show that a person who is silent agrees to a proposition and has given legal consent)