Bottom 贝 is a pictograph of a cowrie shell, the earliest form of Chinese money. As the indexing radical it carries the meaning: trade and commerce are matters of exchange. The same radical groups 贸 with 财 (wealth), 货 (goods), and 购 (buy).
Top 卯 supplies the sound, mǎo drifting to mào. Originally it pictured a slaughtering block split open; here it serves purely as a sound clue. The same phonetic frames 茂 (lush), 冒 (to emerge), and 帽 (hat).
