With each level of this Perk, you will get +20% bonus when checks are made against the Outdoorsman skill, for purposes of determining if the player receives dehydration damage (Desert random encounters) or damage from a rockfall (Mountain random encounters).
The Outdoorsman skill itself does not increase.
In unpatched Fallout 2, this Perk give you +40% bonus to Outdoorsman instead of 25%. [verified]
In Fallout it is bugged so that damage from dehydration does not ignore the player's armor[verification needed], and the damage from rockfalls is negligible to begin with (and is easily absorbed by almost all armor), making this Perk essentially useless in that game.
This perk doesn't exist in the G.E.C.K. but an image of a Vault Boy named "perk_survivalist" can be found in the game files and proves that the perk was planned. Otherwise, benefit, prerequisites and description are unknown.
The image for this perk in Fallout, Fallout 2 and Fallout Tactics is obviously inspired by Sylvester Stallone's character John Rambo, from the movies First Blood; Rambo: First Blood, Part Two; Rambo III and, more recently, Rambo.