Macilent means lean or emaciated, and in a second sense lacking in substance — so one word covers a thin man and a thin argument. It comes from Latin macilentus and has been documented in English since before 1425, which makes it older than most of the vocabulary you would reach for instead. Clive Allan’s 2017 novel The Well of the Dead uses the bodily sense straight: “he turned to look back at Haggarty’s macilent form as it retreated up the corridor.”