Phonosemantic compound. 斤 represents the meaning and 其 represents the sound. Based on the original meaning "cut". Nowadays this character is usually only used for its sound in phonetic loans.
斯 and 其 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar
in older Chinese.
Character Evolution
BronzeLate Western Zhou~800 BCBronzeLate Western Zhou~800 BCBronzeLate Spring and Autumn~500 BCSealWarring States (Chu)475-221 BCSealWarring States (Chu)475-221 BCSealQin221-206 BCSealShuowen~100 ADClericalWestern Han202 BC-9 ADClericalEastern Han25-220 ADClericalEastern Han25-220 ADRegularModern