Chapter Seven begins by talking about the massive impact having Ikemefuna living in his household has had upon Nwoye. Nwoye and Ikemefuna now spend their evenings together with Okonkwo in his hut, listening to him tell them tales of tribal legends, battles and hunting stories. It appears that Ikemefuna has really helped Nwoye to become more "manly" in the eyes of the village and of his father, Okonkwo. One of the ways that the narrator shows how Nwoye has developed as a man is through the following situation:
Nothing pleased Nwoye now more than to be sent for by his mother or another of his father's wives to do one of those difficult and masculine tasks in the home, like splitting wood or pounding food. On receiving such a message, through a younger brother or sister, Nwoye would feign annoyance and grumble aloud about women and their troubles.
Thus we see in the situation you have highlighted that Nwoye feigns annoyance because it gives him an opportunity to demonstrate how manly he has become and how he is developing into a grown-up male.