This better be good

Hello Paco

I take it for an ellipsis of "this had better be good", i.e. "I expect great things from this".

I think the "had" has slowly worn away into a wafer-thin "d" and then slipped down the gullet of the speaker.

MrP

Usually the context would be something like this: you have forgotten something important, arrived very late for an appointment,or have failed in some job you were supposed to do.  As you prepare to offer your explanation/excuse/apology, your wife/boss/co-worker says, "This (had) better be good!" meaning "You had better have a really, really good excuse this time, because I'm out of patience and you're in big trouble!"

Another possible context - you have persuaded your wife to go to a sporting event, a movie or something else that she really isn't interested in.  As it begins, she says "this had better be good" - meaning "if I don't like it, you're going to get all the blame for a ruined evening!"

Yes, I think it's almost a veiled threat -- "This better be good, or else!"  ("Or else you're in trouble!")

Another possible context - I buy a lot of very expensive ingredients and spend all day in the kitchen cooking a new dish.  As I serve it, I say "This better be good!" with the implication being "It was a lot of work and expense - I hope it wasn't all wasted!"