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.斯 /*[s]e/, /*[s]e/, /*se/ 其 /*gə/, /*gə/