Skype for Mac OS X: how to be invisible for a specific contact

Currently, you cannot set invisible for a specific contact or a specific contact list.

However, there is a work around: You can block the contact when you do not want him to see you are online (and you can unblock him when you want him to see you).

When you block a contact, he will not be able to see your online status, call you, or send you messages, and they will receive no notifications, which we leave him clueless about your action of blocking/unblocking him.

See how to block/unblock a contact in Skype for Mac.