宵小 (xiāo xiǎo) literally combines 宵 (xiāo), meaning 'night' or 'evening', and 小 (xiǎo), meaning 'small' or 'petty'. Historically, it referred to thieves who operated under cover of darkness—hence 'night-time petty thieves'. Over time, the term broadened metaphorically to describe any despicable, underhanded, or morally contemptible people—especially those who act secretly, maliciously, or with low cunning. It carries strong negative connotations and is often used in formal or literary contexts, such as news reports, political commentary, or moral essays.

The word functions exclusively as a collective noun and is rarely used in casual speech. It is typically preceded by modifiers like 一群 (a group of), 这些 (these), or 专事 (those engaged in), and seldom appears in plural form since it already implies plurality. Though its etymology references nighttime activity, modern usage focuses entirely on moral judgment—not timing or literal thievery.