It's a common idiom, not slang ("slang" suggests that only certain groups of people use this expression). The meaning is "doubter" or "someone who typically doubts everything they hear". The "Thomas" part comes from the Christian Bible, as John has explained.

it's a person who does not believe whatever you tell them. They will always seek proof and evidence. It comes from the bible where St Thomas refused to believe christ (jesus) was dead, had been crucified, until he had received proof of it. EDIT: I am not religious I made a small mistake, Thomas did not believe Christ had risen. He refused to believe it until he could fell and touch Jesus' wounds himself. A doubting Thomas could also be someone who is habitually pessimistic.

It was a little difficult to find an example sentence. But I found one after sleeping on it. If England win the world cup in Russia this year. When they get/arrive home they can hold the cup up high, when on the victory bus. And say. "to all you doubting Thomas's who though we could not win +(some very bad football language)"

John's answer is goood, but my understanding is a little different form the way his answer reads. For any particular question, a doubting Thomas is someone who doubts this one question. It may be that they need rpoof for everything, but not implied as true. . A: You have a good answer below. B: Maybe. A: Do we have a doubting Thomas here?

informal language: grass=marijuana