Fetching details from the dictionary...
to grumble; to gripe; to complain
友人に愚痴って少し楽になった。
I vented to a friend and felt a little better.
彼女は飲み会で上司のことを愚痴った。
She complained about her boss at the drinking party.
あの同僚は、どんな些細なことでもすぐに愚痴る。
That colleague always grumbles about even the smallest things right away.