I made it up. I blame the Adequacy editors for not inserting an appropriate correction.
Actually, you didn't make it up. Here's the definition:
plangent /"plandZ(<schwa>)nt/ a.E19. [L plangent- pres. ppl stem of plangere beat (the breast), strike noisily: see -ENT.] Esp. of the sound of the sea, waves, etc.: reverberating, mournfully resonant. Also gen., loud-sounding, strident; plaintive, sad.F. W. FARRAR The mingled scream of weltering tempest and plangent wave.P. FARMER Stan stood improvising for me alone his plangent, melancholy music.P. LARKIN Poem after poem begins in her peculiarly plangent way, like a hand swept across strings.plangently adv. E20.