Here are the steps that you can take to hide your online status on WhatsApp and chat secretly.
Open the app on your smartphone and make sure that it is updated.Source: BGR
Go to the "Settings" category in the top right cornerSource: BGR
Now tap on the 'Account' option from the drop downSource: BGR
Scroll down and tap on the Privacy optionSource: BGR
Now click on 'Last seen & online'Source: BGR
Select the "Who can see me online" optionSource: BGR
Select the contacts as per your preference.Source: BGR