Is “unlikely liar” a correct way to descibe a person?

Is unlikely liar a correct way to describe a person who is unlikely to lie?

Solution 1:

Dictionary definitions and example sentences should be given, but there will then be uncertainty as to which sense of 'unlikely' is the default one here.

Used of a person, I'd say the default sense here is as given among the senses at Macmillan:

unlikely ... ... 2 not typical ... unusual in an interesting way; unusual, special, unique...

He’s a very unlikely romantic hero.

[reformatted]

So the required 'not really likely to be true' sense is at best in doubt here (though context almost always helps to disambiguate different possible polysemes). The sense is rather 'unexpected; not a natural'.

'It is unlikely that he's a liar' gives the required sense.

Solution 2:

No. Unlikely as an attributive adjective actually accepts that its associated noun is true. Dictionaries don't really help with nuances like this:

unlikely

Not likely to happen, be done, or be true; improbable.

‘an unlikely explanation’
‘it is unlikely that they will ever be used’
[with infinitive] ‘prices are unlikely to change’

— Oxford via Lexico

An unlikely explanation is an explanation, albeit unexpected and improbable.

An unlikely teacher is someone who teaches, although you didn't really expect them to do that.

An unlikely liar is a liar, however surprising it is that they are.