This is the adverb of impecunious. If someone lives impecuniously, they live with very little money, often for a long time. The word is rare and rather bookish; most people never say it. When it does appear, it is often slightly humorous, as if the writer is putting on a fancy coat to talk about being poor. You are far more likely to meet the adjective impecunious than this -ly form.

窮困地、囊中羞澀地（極書面）