The undesirable effect achieved when one tans their front and back forgetting the all important side area. Most common in larger sunbathers due to the increased surface area and best effect achieved by topless flat chested people predominantly males. Red tannards are particularly sore.
Mostly, clankin' tankards just means getting sloshed - most likely in a medieval setting. However, the phrase could absolutely be a euphemism for something (like getting railed in a sundress by an Hobgoblin, or getting boofed by a Bugbear).
"Avast ye! Hast thou been Clankin' Tankards at the Gasping Gander?? I can see the juicesall overyer face!"