When you do something glancingly, it's indirect or subtle. If your friend only glancingly mentioned their peanut allergy, you might completely forget about it.
When you're at an old hotel hearing spooky sounds at midnight, you may suddenly remember the innkeeper glancingly referring to "our ghost." Doing something glancingly means you just barely touch on it. Physically, this can be a blow that comes from the side and moves off at an angle: "The knight struck glancingly, his sword deflecting off his foe's armor." Glancingly is from the Middle English glacen, "to graze," and its root, meaning "to make slippery."