This achievement is for unlocking the Ankh backpack.
Method 1: To unlock this in the main game:
First you must unlock the Ball backpack
in Robot World*
by purchasing it from the shop. Bring the Ball to Sally and press use to enlarge the ball. Bounce the ball against Sally's face several times without letting the ball hit the ground. The Ankh will appear where the ball lands after you meet this requirement.
Method 2: There may also sometimes be daily challenges with the Ankh available in them.
As in one of the items given at the start. In which case, walk forward and pick up the Ankh.
Method 3: Thanks to Danyaelus ThePaleHorseman
there is also another method if you'd like to try it out instead:
You can use and abuse a secret passage in the death area which will give you a random head, weapon, and backpack repeatedly until you get the ankh (and all other items if you wish). It's long, so details in the comments/other solution for now. Give ThePaleHorseman
an upvote too!
*note: it may be possible to find in shops in other worlds, so might be worth your time to check along the way.
Although most sources state it must be robot world.
Posted with permission on behalf of Uh iCanExplain
(accidentally deleted original solution)