This is the 15291st most frequent Russian word.
поссорившихся
"Поссорившихся" means "those who have quarreled" or "having quarreled."
Here, "поссорившихся" is used as a past participle derived from the verb "поссориться," denoting the state of having quarreled. It is used to describe "друзей" (friends) who have had a quarrel.
Он помог примирить поссорившихся друзей.
He helped reconcile the quarreled friends.
In this sentence, "поссорившиеся" acts as a past participle, modifying "соседи" (neighbors) to indicate their previous state of having quarreled.
Поссорившиеся соседи решили больше не враждовать.
The quarreled neighbors decided not to feud anymore.
The term "поссорившихся" here functions as a descriptive participle modifying "политиках" (politicians) to specify that they had a quarrel.