Nik Wallenda -- Grandma NOT Impressed with Sunday's Death-Defying Tightrope Walk

Daredevil Nik Wallenda will flirt with death Sunday in Chicago -- traversing 2 buildings -- but it's still not enough to impress his grandma.

Nik will step on a high wire Sunday which will be attached to 2 skyscrapers -- 2 city blocks apart. It will be an uphill walk -- the equivalent of 8 stories, and Nik will be blindfolded for part of it.

It's not enough to wow or scare his 87-year-old Oma, who watched Nik practice this week. Oma herself was a performer in the Wallenda family -- her thing was hanging by her teeth on a high wire.

Nik tells us he spiced things up with a few tricks ... backwards blindfolding and kneeling on the wire. She still wasn't blown away.

Granny should know ... Nik will walk without a net or harness Sunday. It will air live at 7 PM ET on Discovery.