居 and 古 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.居 /*k(r)a/, /*ka/, /*k(r)a-s (based on rhyme evidence)/ 古 /*kˤaʔ/
live, dwell, reside, sit; to reside; to be (in a certain position); to store up; to be at a standstill; residence; house; restaurant; classifier for bedrooms
