Lethal means able to kill. A lethal dose, lethal weapon, potentially lethal.
It is used of substances, weapons and conditions rather than of people. Lethal dose is the exact toxicological term, and LD50 is the dose that kills half a test population. Non-lethal and less-lethal are the policing terms for weapons meant to incapacitate. Informally, lethal is used admiringly — a lethal serve, a lethal sense of timing — meaning devastatingly effective. Note lethal injection, and that lethal is stronger than dangerous and more precise than deadly.
1adjectivelethalAble to kill.
“The snake's bite can be lethal if it isn't treated quickly.”
synonyms: deadly, fatal
2adjectivedevastatinglyDevastatingly effective.
“She has a lethal backhand and knows exactly where to put it.”
lethal is a quality of deadliness.
Also accepted in games: deadly, fatal
The Latin behind it is simply death. Lethal carries death in it and always has.
hooks ontodeath
Latin letalis comes from letum, death. The h in the English spelling was inserted in the sixteenth century by writers who wrongly thought the word came from Greek Lethe, the river of forgetfulness.
Latin
lētālis — deadly, mortal, fatal
English, today
lethal
Built from
Latin lētālis (“deadly, mortal, fatal”), improperly written lēthālis, from lētum (“death”), improperly written as lēthum, from a supposed connection with λήθη.
Etymology from Wiktionary, CC BY-SA
Practised in Sharper Adjectives