Why are they called tank tops ?

Because wearing one immediately enlists you in the army

Today I learned… [image deleted]

Lol sorry I can’t resist on a tank it needs easy movement of the guns.
Maybe likewise for your comfort.

They do show off your "guns".

neither do tanks, depending on the threat 🤷‍♀️

Cause it raises loss of tank to top side

Slingshot

Tank outfits used for going in water back in those days.

It's rhyming slang, tank rhymes with wank. It's because only wankers wear them 🤭🥁