In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Saaleh Al-Fawzaan (may Allaah preserve him) said:
[مكر (plotting)]: It is to deceive the people by showing them the opposite of what you conceal. You show them that you are sincere and truthful to them, whilst you are the opposite of that, in order to harm them. Plotting is to harm another person in a hidden manner which (the one being harmed) does not realise. This is not permissible.
[Abridged & slightly paraphrased. Source: It’haaf At-Tullaab Bi-Sharhi Mandhoomah Al’Aadaab’ page 101]