ASIC-resistant is just a buzzword for shitcoins.
ASIC, as the name stands for, is an Application-Specific Integrated Circuit.
So, there's nothing in theory you could do to make an algorithm "resistant" to an ASIC.
Given any algorithm, you can design an ASIC for it. An ASIC is simply hardware made to run a specific algorithm as efficiently as possible.
One possible way to make something ASIC "resistant" would be to basically change the algorithm frequently, so that you don't have the time to design and build an ASIC. But that doesn't make too much sense to me.