Sometimes, in our investigations, we come across words such as <raspberry>, <skillet>,<yeoman>, etc., which, when investigated, lead to uncertain or unknown etymologies (see below), and few, if any, related words (unless skillet is related to skill, which it doesn't seem to be). What do you do in those cases?

yeoman: c.1300, "attendant in a noble household," of unknown origin

raspberry: 1620s, earlier raspis berry (1540s), possibly from raspise "a sweet rose-colored wine" (mid-15c.), from Anglo-L. vinum raspeys, origin uncertain, as is the connection between this and O.Fr. raspe, M.L. raspecia, raspeium, also meaning "raspberry." One suggestion is via Old Walloon raspoie "thicket," of Germanic origin.
skillet: c.1400, perhaps from M.Fr. esculette (Mod.Fr. Ă©cuelle), dim. of escuele "plate," from L. scutella "serving platter" (see scuttle (n.)); or formed in English from skele "wooden bucket or pail" (early 14c.), from a Scandinavian source (cf. O.N. skjola "pail, bucket").