忽

A phono-semantic compound:

心 (heart, mind) — semantic component, relates to attention, awareness, mental focus

勿 (do not; must not) — phonetic component

The original sense is “the mind failing to attend”, which naturally gave rise to neglect, carelessness and by extension something happening so fast it escapes attention, suddenly.

홀연 (忽然) — suddenly; abruptly

소홀 (疏忽) — negligence; carelessness

경홀 (輕忽) — treating lightly; disdain

홀시 (忽視) — to ignore; to disregard

Adverbial usage:

忽 alone can function as an adverb meaning “suddenly” in classical and literary Chinese.

Numerical usage:

In classical and technical systems of measurement:

忽 = 10⁻⁵

This belongs to a traditional Chinese decimal hierarchy:

分 → 厘 → 毫 → 絲 → 忽

Related characters:

忽 — neglect, sudden

急 — urgency, haste

突 — suddenness (external event)

卒 — sudden, unexpected (often death)

忽 emphasizes mental unpreparedness, not merely speed.