Originally the same character as 于, depicting a musical wind instrument with a reed, which is now written as 竽. The current meaning is a phonetic loan.
Character Evolution
Oracle Bone~1250-1000 BCOracle Bone~1250-1000 BCOracle Bone~1250-1000 BCBronzeLate Shang~1100 BCBronzeLate Shang~1100 BCBronzeLate Western Zhou~800 BCBronzeMid Spring and Autumn~600 BCBronzeLate Spring and Autumn~500 BCBronzeWarring States475-221 BCBronzeEarly Warring States~400 BCBronzeMid Warring States~300 BCSealWarring States (Chu)475-221 BCSealShuowen~100 ADClericalQin221-206 BCClericalWestern Han202 BC-9 ADClericalWestern Han202 BC-9 ADClericalEastern Han25-220 ADClericalEastern Han25-220 ADRegularModern