A man who rarely speaks the truth:

A man who rarely speaks the truth: Correct Answer Liar

The one word substitution is Liar.

Crook : informal a person who is dishonest or a criminal.
Liar : a man who rarely speaks the truth
Scoundrel : a dishonest or unscrupulous person; a rogue.
Hypocrite : behaving in a way that suggests one has higher standards or more noble beliefs than is the case.

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