The phrase "too good to be true" is a modern English idiom formed by combining fundamental Germanic vocabulary. Its logic is purely syntactic and semantic, not morphological, creating a self-contained descriptive unit. The expression captures a universal human skepticism, suggesting that an offer or situation of excessive perfection likely contains a hidden flaw or deception. Its persistence in the language underscores a pragmatic cultural wisdom, warning against naive credulity by framing extreme goodness as a potential indicator of falsehood.

This is a fixed adjectival phrase composed of four common English words. It functions as a single lexical unit (a compound adjective). The individual words are: 'too' (adverb, from Old English 'tō'), 'good' (adjective, from Old English 'gōd'), 'to' (particle, from Old English 'tō'), 'be' (verb, from Old English 'bēon'), and 'true' (adjective, from Old English 'trēowe').