Alethic is derived from the greek word alēthikos, from alētheia truth (fr. alēthēs true, fr. a- 2a- + -lēthēs, fr. lēthē forgetfulness) + -ikos -ic.
In the discipline of modal logic, “alethic” entails being of or pertaining to the various modalities of truth, such as the possibility or impossibility of something being true.
It is where the word lethal is derived, from Latin lētālis (“mortal, deadly”), improperly written lethalis, from letum (“death”), improperly written as lethum, as associated with Greek lethe, meaning “forgetfulness”.
Interestingly, that which is “true” is “not lethal”, or “not forgetful”. Death is oblivion, or total forgetfulness. Truth is the anti-measure of forgetfulness. That which is true appears lasting and memorable.