打招呼

打招呼:

Literally means:

greet; give notice beforehand

Actually means:

It means extending greeting by verbal or nonverbal means.

Metaphorically, it means giving somebody notice of something beforehand, or asking somebody to help in secret so as to make things proceed smoothly or to succeed.

见面时用语言或动作表示问候。

比喻就某事通知某人，或请某人暗中照顾或帮忙，使事情能够顺利进行或成功。