This seems to be my day for Brahms. If anyone still cares--I recently saw a translation (in the liner notes to Twilight and Innocence, an album by Heidi Grant Murphy, soprano, and Kevin Murphy, piano). They translated "Naeglein" as "carnations". So Daniel Malena was on the right track. I guess the connection is that carnations smell like cloves, and cloves look like nails. After all, one English name for carnations--clove pinks--comes from the French word for nail--clou.