друг на друга

• ДРУГ ДРУГА <ДРУЖКУ coll>; ДРУГ ДРУГУ <ДРУЖКЕ coll>; ДРУГ НА ДРУГА <НА ДРУЖКУ coll>; ДРУГ О ДРУГЕ <0 ДРУЖКЕ coll> и т.п.

[NP (has no nom case); sing only; fixed WO]

=====

⇒ each of two or more (people, things etc) does sth. to the other(s) (used to indicate a reciprocal action or relationship):

- (to <about, against, near etc>) each other (one another);

- one after (against, over, without etc) the other;

- [in limited contexts] each other;

- one another.

     ♦ Они впервые стояли рядом - зять и свояк. За столом они не сказали друг дружке ни слова (Абрамов 1). It was the first time they had stood side by side-as brothers-in-law. They had not said a word to each other at the table (1a).

     ♦ "Доктор! Решительно нам нельзя разговаривать: мы читаем в душе друг у друга" (Лермонтов 1). "Upon my word, Doctor, it's impossible for us to talk together. We read each other's minds" (lc).

—————

← "Друг" is the short form of the adjective "другой".