drop in—
1. collapse inwards; give way:
- The roof of the tunnel hasn’t been properly supported; it shows signs of dropping
2. (coll.) visit a person casually:
- He usually drops in at my place on his way home.
fall in—
1. fall accidentally into smth.:
- The child was near the river and I was frightened he would fall in.
2. = drop in 1:
- They are afraid that the whole building will fall in on them.
3. (Military) form into ranks:
- At first light Chelmsford ordered his men to fall in and the column marched away.
4. (legal) expire:
- As the leases of these old properties fall in, they are being pulled down to make way for office blocks.
5. agree:
- Once the chairman had stated his decision, the rest of the committee fell in.