When, whether due to overly-restrictive clothing, uncomfortably low temperatures, or the use of stimulants, a man's penis retreats almost entirely into his body. In extreme cases it's possible for even an uncircumcised male's member to completely disappear from view, leaving only the outer skin of his penis bunched up like the neck skin of a turtle hiding its head in its shell.
"Man, these running short are too tight. When I got to the bathroom it took me a minute to take care of the bashful turtle so I could take a leak."
"I swear I'm never hooking up on a ski trip again. When his pants came off all you could see was his bashful turtle."