drop down—
1. come down suddenly:
- I heard a noise from the well and saw that the cat had dropped down.
2. bend downwards; sink down:
- When they saw the pursuers they dropped down to take cover.
3. (coll.) pay an unexpected visit:
- Let’s drop down to his summer home and see if he’s there.
fall down—
1. = drop down 1:
- The hunter fired and the deer fell down dead.
2. (of a building, etc.) collapse:
- The steel scaffolding fell down and narrowly missed a couple of bystanders.
3. (of a project, etc.) be unsuccessful; fail:
- His plan fell down when it proved too costly.