Install Pidgin Off-The-Rectord Messaging plugin:Run Pidgin, go to Tools > Plugins and check "Off-the-record Messaging", then click on "Config" (at the bottom) and then on the "Generate" button to generate a fingerprint.

3. When entering a conversation you will see a new button on the upper right side of the input. Click on it and then select "Start private conversation" whenever you want to chat in private mode.