Definition
A lethal weapon is any instrument capable of causing death or serious bodily harm. In modern American criminal law, the term functions as a near-synonym for "deadly weapon" and describes objects used or threatened in a manner likely to produce fatal or gravely injurious results. The phrase appears most frequently in criminal statutes governing aggravated assault, robbery, and weapons offenses, where the presence of a lethal or deadly weapon elevates the severity of the charge.
The concept covers two broad categories: (1) instruments designed for the purpose of causing death or injury, such as firearms and knives; and (2) objects not inherently dangerous but used in a manner capable of causing death, such as a blunt instrument or even an automobile. Whether a particular object qualifies depends on the facts of the use, not the object's design alone.
Common Language
Modern common usage (Wiktionary): A weapon that is capable of causing death; also used colloquially and in popular culture to describe anything highly dangerous.
Historical common usage (Webster's 1913): Not separately defined; "lethal" appears as an adjective meaning deadly or causing death, from the Latin lethalis.
The common meaning and the legal meaning are superficially aligned — both denote something capable of killing — but the legal context introduces precision the common usage lacks. In criminal law, "lethal weapon" carries jurisdictional and contextual weight: courts must determine capability and intent of use, not merely the object's inherent nature. A shod foot or a piece of lumber has been held to qualify. The casual common usage does not capture this functional, fact-specific legal analysis.
Common Confusion
LETHAL WEAPON vs. DEADLY WEAPON: These terms are functionally interchangeable in most modern American jurisdictions. "Deadly weapon" is the dominant statutory term in U.S. criminal codes, while "lethal weapon" appears less frequently and carries historical roots in Scots law. Researchers should not treat any difference in phrasing as a meaningful substantive distinction without checking the specific jurisdiction's definitions. The historical dictionaries' exclusive framing of "lethal weapon" as a term of Scots law understates how broadly the phrase has been absorbed into general criminal law usage.
LETHAL WEAPON vs. DANGEROUS WEAPON: Some statutes distinguish between deadly/lethal weapons (capable of causing death) and dangerous weapons (capable of causing serious bodily injury short of death). The line is often blurry in practice and varies by jurisdiction.
Why It Matters in Research
The historical law dictionaries — Black's first and second editions and Rapalje & Lawrence — define "lethal weapon" almost exclusively by reference to Scots law, offering little guidance for American practice. This framing is a research trap: a researcher relying solely on these sources might conclude the term is foreign or archaic, when in fact it circulates freely in modern American criminal law as a functional equivalent of "deadly weapon."
The operative research question in almost every modern context is not "what is a lethal weapon" in the abstract, but how the jurisdiction's statute defines the term and how courts have applied it to specific objects and conduct. Many states define "deadly weapon" by statute and then interpret "lethal weapon" by analogy when it appears in older case law or common-law charges. Tracing how courts in a target jurisdiction have moved between the two phrases is essential.
Historical sources that pre-date widespread codification of weapons offenses will use "lethal weapon" more frequently as a term of art. Post-codification sources trend toward "deadly weapon." Researchers working with nineteenth-century case law should be alert to this shift.
The Black's 2nd edition entry is notable for its citation to State v. Godfrey, 17 Or. 300, 20 Pac. 625 — one of the few anchors connecting the historical dictionary treatment to actual American judicial usage and worth examining as an early American court's engagement with the concept.
Historical Dictionary Support
All three historical sources — Black's (1st Ed.), Black's (2nd Ed.), and Rapalje & Lawrence — are in complete agreement: "lethal weapon" is defined as a deadly weapon, and the term is situated within Scots law. The entries are nearly identical, with Black's 2nd edition adding only the Godfrey citation. This convergence suggests the compilers viewed the term as primarily foreign in origin and did not treat it as a standalone American legal concept requiring independent analysis.
What the historical dictionaries collectively miss is the term's absorption into American criminal law vocabulary. They offer no discussion of the functional test for what makes a weapon "lethal," no treatment of the instrument-vs.-use distinction, and no acknowledgment that courts routinely apply the concept to objects far removed from conventional weapons. Researchers should treat the historical entries as a starting point that identifies the term's genealogy, not as a working definition for modern American criminal law purposes.
Jurisdictional Note
American jurisdictions overwhelmingly prefer "deadly weapon" in criminal statutes, and that term carries the bulk of the case law. A handful of states and older common-law formulations use "lethal weapon" interchangeably. In any jurisdiction, the key is whether the weapon was used or threatened in a manner capable of causing death — a factual and contextual inquiry that statutory definitions shape but do not always resolve.
Encyclopedia Cross-Reference
The Law Mind Criminal Law Encyclopedia: Assault with a Deadly Weapon (criminal_39)
The Law Mind Criminal Law Encyclopedia: Weapons Offenses — Unlawful Possession (criminal_109)