嘢 is a Cantonese-specific word meaning 'thing, matter, stuff'. It's used very commonly in spoken Cantonese and informal Cantonese writing, equivalent to 東西 (dōngxi) in Mandarin. You'll hear it in phrases like 好嘢 (hou2 je5, 'good stuff') and 冇嘢 (mou5 je5, 'nothing'). It has no standard Mandarin pinyin; the input pinyin '#' is used to indicate this.