Backstab was once a skillcard gained by joining the “Assassin's Guild” as Thief, back when guilds existed. This explains why it is colored gray and why Thief cannot get it from any normal item pools.

You Choose, You Lose:

• 2/7 chance to be offered at the floor 2 shop
• 1/9 chance to be offered at the floor 3 shop
• 1/8 chance to drop as the floor 4 equipment
• 1/3 chance to drop in the floor 5 shop
• Total chance to drop: ≃85.5%

Thief in the Night, Finders, Keepers, The Elimination Round (Thief):

• 20.25% chance to be offered as a trade for any of Poison Needle, Crowbar, Lockpick or Dagger