I've never heard of such a term being used around here, so I would assume he's either talking about a military tank or a storage tank. I get that your definition may somewhat fit with the one I provided, but honestly, it just hardly fits. On top of that, you're misleading him into thinking that's a real thing.
This game has a plethora of slang terms for certain builds and playstyles.
Tank builds have been around for a bit, and as described a page back, it's a high defense, high BFE/BFA player that can still succeed in hitting you, with little success in you hitting back.
The best set depends on what stats you want to boost the most. The ones with the highest stats though are the ones given out as a reward for the rancor war seasons depending on what rancor level you achieve or special events such as moths and feathers.