The speaker looks back on a relationship where very little was said aloud. "We loved in the fewest words" sets up a love built on restraint, and the two odd words that follow, "fickly" and "demandable," seem to show what those few words were like. "Fickly" hints at changeability, as if the feeling wavered. "Demandable" suggests something that could be asked for or claimed. Both feel slightly off, almost invented, which fits a language the couple made up between them or never quite mastered.
The second stanza pulls toward intimacy. The two are "loath to speak with one mouth," which could mean they were reluctant to merge into a single voice, or reluctant to speak at all. Then the image of a mouth "red and full of evening" shifts the poem from words to the body. The mood is quiet, dusky and a little wistful. The poem suggests that what mattered in this love lived in silence, touch and the fading light rather than in anything said.