(after a suppositional clause) in that case; then; (after a clause of
Phonosemantic compound. 京 represents the meaning and 尤 represents the sound. Based on the original meaning "high place". The meaning of this character has shifted over time and now means "achieve".
《說文》：“就，就高也。从京，从尤。尤，異於凡也。，籀文就。”
high place