noun
gripe session; grievance session
A mutual exchange of complaints, often informal and among peers. The compound 零し合い emphasizes the reciprocal nature of the complaining.
飲み会が零し合いになってしまった。
The drinking party turned into a gripe session.
同僚と零し合いをしてストレスを発散した。
I let off steam by having a grievance session with a colleague.
愚痴 is a general term for complaints or grumbling, while 零し合い specifically refers to a mutual exchange of complaints between people.
零す means 'to spill' or 'to complain'. 零し合い is the noun form of the compound verb 零し合う, meaning to complain to each other.
Compound of 零し (noun form of 零す 'to complain') + 合い (noun form of 合う 'to do mutually').