Right side is
吾 'I, my' (a classical first-person pronoun), supplying the sound exactly: wú matches wú. The phonetic role is straightforward; the semantic flavor is incidental. In Cantonese,
唔 has been borrowed as the negative particle (m4, 'not'), but in Mandarin it stays a thinking-hum. Same phonetic in
悟 to realize and
梧 wutong tree.