falsus

Falsus, falsa, falsum is the perfect participle of fallo, "to deceive", pressed into service as an adjective. That participial origin is the whole trap: it is passive in form, so its first sense is "deceived", and only by extension "deceptive". A falsus homo may be a man who has got it wrong rather than a man who is lying, and the sentence has to tell you which.

The neuter falsum is a noun, "a falsehood", and the ablative falso works as an adverb meaning "wrongly, mistakenly" - falso accusatus, "accused on a false charge". Modern English "false" has narrowed to "untrue" and lost the other half, which is why translations of this word so often flatten it. The family gives fallacy, fail, fault, default and, through Italian, falsetto.