Refers to a type of suit used for infiltration purposes, along with any other type of clandestine act. A sneaking suit will usually be colored so that it can meld with scenery, and will also allow cargo-space via equipped pouches.
"Solid Snake's sneaking suit varies from different Metal Gear games. The most popular sneaking suit for Solid Snake is the one from Metal Gear Solid, which was a grey BDU with a black vest, which was also equipped with kneepads, elbowpads, cargo-space, and was especially modeled with state-of-the-art polythermal technology to prevent hypothermia and thus keep the wearer nice, dry, and warm. Supposedly it's hard to move in the suit though, so training may be necessary."