Best definition
to complain in a childish fashion by questioning the cause or reason for which something occurred

A: “Why are we here? Why are we doing this? Why can’t we go home?”

B: “Stop whyning!”
Whyning: define #2
frivolous or tedious hostile questioning done to avoid acknowledging something one disagrees with
I showed him the new app worked better for everything he needed, but he started whyning about the features because he didn’t understand the new menu.