surname Yu; Taiwan pr. [Yu2]
Originally a variant form of 烏, which is a pictograph of a crow. The current use as a function word is a phonetic loan.
《說文》：“烏，孝鳥也。象形。孔子曰：‘烏，呼也。’取其助气，故以為烏呼。，古文烏，象形。於，象古文烏省。”