The term Bulletproof (which I see is a registered trademark) has been adopted by countless low carbers like Coca Cola has been for that flavour of soda.

Quality butter, virgin coconut oil (a natural medium chain triglyceride), good coffee / tea, natural vanilla, and heavy cream make a kick ass drink... I pack it in my thermos on winter rides and also make it with quality chai which is closer to the original Tibetan po cha which has been served in Tibet for millenia and was / is a major source of calories.

If I could get Yak butter I'd be all over that...