Kourtney and Mason were out on the town!

The mom of three took her oldest to visit an office building -- and while that may seem mundane afternoon activity for some, it looks like Mason Disick was having a blast!

Kourt went casual in a black crop top sweatshirt with grey tank underneath. Of course, the 36-year-old showed off her curves in skintight leggings -- but sadly mama K must have forgot that when she wears painted-on leggings her panty lines show for serious!

She better get that undie situation figured out because now that Justin Bieber's single again, the Biebs may come aknockin' ...

Better get a thong, girl!