Are a type of swimwear for men that are skintight like briefs or trunks but extend down your legs to just above your knees. They are primarily used by competitive and/or semi-serious swimmers. Often in your local municipal pool, when it's taken over for the club swimmers, most of the gents will be wearing these.
Although in my view, anyone who swims regularly or semi-regularly ought to get themselves a pair. You don't have to get the super-duper ones made out of lycra, kevlar, and sharkskin, but I would still recommend them.
The reasons I would recommend them over any other form of male swimwear even for semi-casual swimmers like myself is that they're so. freaking. comfortable.
They might not look it. But they are.
Unlike shorts they don't flap about, get air trapped in them leading to embarrassment when the air is released with a whoosh of bubbles not unlike a fart, or leave your twig 'n' berries flapping about in the breeze.
Unlike square leg trunks they don't squash your bollocks or ruck up in your groin or arsecrack after prolonged use.
And unlike budgie smugglers they don't cut off your circulation and/or look ridiculous on anyone other than a snake-hipped male model type.
No, a pair of jammers is ultra comfortable, in my humble opinion, even if you're a big chap like me. They support your lunchbox perfectly without squashing it, they don't ruck up, and they don't cut off your circulation. What is more, I am informed that even a big gent like myself looks quite handsome in a pair (though this was by a gentleman with impeccably groomed hair who was giving me a glad eye from the poolside, so I'd take that with a pinch of salt.)
Which is why anyone who swims regularly, even if it's not in competition, ought to get themselves a pair.
(Node 21 of 30 of my IRON NODES).