The speaker invites someone to a meeting, and the invitation is oddly unguarded. "Meet me in thin arms" suggests an embrace that is fragile or not quite enough, yet the speaker asks for it anyway. Then comes a concession: go ahead and tell me it's only a scam, a way to sell me a trick or lead me around by a ring in the nose. The speaker seems fully aware they might be fooled, and is willing to accept the lie.
The last two lines turn on a quiet admission. "I know how true / rain would be." Rain stands for something real, honest and cleansing, and the speaker can imagine exactly how it would feel. That knowledge makes the false offer sadder. The mood is wry, lonely and a little resigned. The speaker would take a flimsy substitute for closeness, while knowing what the real thing would be like, and the poem leaves that ache unresolved.