drop someone a line

(verb) make or keep in contact with someone

Example Sentence:

I’ll start the first part of the project, you start the second, and drop me a line in a few days.

Student 1: I am going back to Brazil.
Student 2: I am going back to Korea.
Student 1: Good luck to you! I hope we will meet again soon!
Student 2: Drop me a line!

The boyfriend is always dropping his girlfriend a line while they are separated.

From the Blogs:

White Collar Criminals: Drop me a line

Media:

Similar idioms:

1. hot under the collar
2. touch base
3. pick up line
4. cross the line
5. drop an F bomb
6. a drop in the ocean
7. don’t be a stranger
8. drop out
9. keep in touch
10. keep someone in line
11. take care
12. beats me
13. peace out
14. bye!
15. put one’s ass on the line