Not believable — describing something that tries to make you believe it, and fails. An unconvincing excuse, an unconvincing smile, an unconvincing argument, unconvincing evidence. Notice what all of those have in common: each one is an attempt. The word does not simply mean 'false'; it means the attempt did not land. In fact something unconvincing can be perfectly true and still sound wrong, which is why English speakers reach for this word when they want to criticise without calling anyone a liar. That makes it a very useful polite weapon — a reviewer writes 'the ending is unconvincing', a boss says 'I find that explanation unconvincing', and nobody has had to say the word 'lie'. Two extra uses worth knowing. In sport, a team can win unconvincingly — they got the result, but nobody believes they are strong. And in acting, an unconvincing performance means you never forgot you were watching an actor.