Kourtney Kardashian Shoutout to Rob ... Where For Art Thou, Bro? (PHOTO)

Kourtney Kardashian got a little nostalgic about her brother, and she picked a weird way to show it ... posting a pic of him with his hand down his pants.

The photo, showing a skinny Rob Kardashian, is old from when Rob was living at Khloe and Lamar's pad. It was taken about 4 years ago and posted on a fan blog site.

Maybe Kourt just discovered that site, or more likely ... she's doing what all the sisters have been doing in recent months -- posting old pics of Rob, which they frequently delete.

Of course, Rob's been out of the spotlight ever since Kim's wedding last year. So, they're either encouraging him to resurface, or just giving him a public S/O to say ... we still got you, bro.

Regardless ... sweet Bundy pose!