The speaker in the poem has been nurturing wrath against his enemy. “It” stands for the grudge being fostered behind the façade of his smiling face.

The poet employs the literary device of extended metaphor to express the nursling of wrath within one’s heart. The cultivation of angst and malice within oneself has been likened to the nurturing of a plant.  

With proper supply of water and sunlight, a plant grows to become a tree. Similarly, a petty incident sows the seed of wrath in the speaker, who feeds it with tears and malice. With time, the seed of wrath grows and acquires menacing proportion.

So, we see that “sunned it with smiles” is a part of the extended metaphor that compares the nurturing of grudge to the nursling of a plant.

The verb “sun” in the quoted phrase means feeding the speaker’s wrath with the nourishment required for its growth and strength. As the sun provides plants with the indispensable ingredient of sunlight for their growth, spurious “smiles” nourishes the hostility within the poet.

Generally, through smiles we convey our good-will, admiration and respect for others. But the “smiles” referred to in the poem are evil and malignant. They are merely a façade covering the vindictive and venomous feelings being cultivated in the speaker’s mind. They, in a way, help the plant of wrath and vengefulness grow bigger and stronger.