A person who is unwilling participate in what he perceives to be risky, embarrassing, dangerous or foolhardy activities; the term is usually used to denigrate such individuals by those who do participate in the same activities.
Similar usage sometimes to 'coward', 'sissy' or 'momma's boy'.
"Don't be such a jamtart, Bill. The rest of us are all jumping off the cliff. You're such a sissy!"
"Hey, did you notice the Thormplegorp Twins at the theater today? They refused to watch Eat My Shorts. What a couple of jam tarts, eh!"