Readings & Meanings:
I overslept, so it seems I was late for school.
He's slow at walking.
The radical is Shinnyou, the Road Radical. See http://sokasoka.blogspot.com/2008/03/line-them-up-and-choose.html for the introduction to Shinnyou.
The element on the road derives from 犀. (This is a lot easier to see if you look at the older version of 遅: 遲.
犀 currently means rhinocerous, but it really refers to any big, lumbering animal. The 尸 above is "body," and the 辛 below is "needle," or horn.
Shinnyou indicates something to do with movement. How does a big, lumbering, horned animal move? Probably pretty slowly.
I heard this all the time in the Naruto anime, back when it was interesting. Ninja A would take a swing at Ninja B, but Ninja B was too fast and hit Ninja A instead, saying "Osoi!" In this context it means "Slow!" or "Too late!"
You can say it, too, in a non-otaku context. If you tell a joke or say something clever, and there's one friend listening who has no idea what you're saying, but then, a minute after everyone else, finally understands, you can call them "遅い!"