忽

Bottom heart radical - the full underfoot form of 心, three dots curving along a hook (vs the left-side 忄 form). The indexing radical, files 忽 in the family of mental-state chars: 想 think, 念 miss, 感 feel, 愁 worry. Marks 忽 as a state of mind - a heart that flickers and forgets, the source of 'suddenly' and 'to overlook.'

Top 勿 supplies the sound - wù shifting to hū with consonant change (well-attested w/h alternation in this Old Chinese phonetic series). 勿 itself depicts a banner with trailing streamers, hence its prohibitive 'don't!' sense (a flag waved in warning). Same phonetic family: 物 thing, 易 (faintly), 匆 hurriedly. Note 匆/忽 form a near-synonymous pair in 匆忽 'in a rush.'