When your legs are streched, for example when you stand or lie on your back, there is a narrow area, about a finger wide, between where your legs join the body and your genitals on the underside of your torso. What is this part of the body called?
Groin, as the OED defines it, seems to refer to "the depression between the abdomen and the upper thigh", that is, an area on the front of the body and not between the legs, where the area lies that I'm after.
Here is an image in which the area is marked in green: https://i.stack.imgur.com/OFaO3.jpg (NSFW, genitals covered). Please note how the 'string' of the taut adductor muscles separates the groin on the front side of the trunk from the area in question on the trunk's underside. The image shows a female body and the perineum is invisible below the genitalia in the shadow between the buttocks.
https://commons.wikimedia.org/wiki/File:Courageous_Cunt_06.jpg (NSFW, explicit)