Playground Slide, Slide
noun
Remember how 滑る is "to slide"? Then, how 台 is a counter for machines? In this case, this slippery machine is a slide, more specifically a playground slide.
すべりだい
The reading comes from 滑る and 台.
甥っ子の誕生日祝いに滑り台を作ってるんですが、少々予定が遅れ気味なんですよね。
I’m making a slide for my nephew’s birthday, but I’m a little behind schedule.