Someone who is out at a barbeque where the food is plentiful, the drinks are flowing, the music is pulsing, and everybody is out on the dance floor is likely to have a blast at that party. The expression "to have a blast" isn’t as transparent or obvious as many English expressions. Be that as it may, because everybody likes a good time, it’s a popular and common expression likely to crop up in conversation from the most casual to the most professional.
Most of the time, having a blast suggests energetic fun and maybe even a little rule bending. It could very well entail staying out past curfew for young partyers and staying out past the time the babysitter expects for older ones. A blast is such a good time that there’s almost no way to control it or escape its happy, excited clutches. It might even be used as an excuse of sorts, for example, when a student apologizes for a late paper by telling the teacher it was his mother’s birthday, and they were all having such a blast that he couldn’t pull himself away long enough to get his work done.
The expression to have a blast lends itself to sarcasm as well. As a coworker trudges into a meeting that is likely to turn into a confrontation, an office mate might toss off a cheery, “Have a blast!” Someone who has just returned from the world’s worst first date might glumly report to friends that they had a real blast, meaning exactly the opposite.
It’s difficult to imagine how the phrase originated. The word blast has a number of meanings. It can be used to describe a strong burst of air that is usually very cold or a very loud sound. A boss who is very unhappy with an employee will blast her, which means to tell her exactly what she’s doing wrong. “Blast” is a slightly old-fashioned expression of frustration uttered by older people who have just been cut off on the road or opened an unexpected bill.
It’s not clear how any of these uses of the word blast might contribute to the expression’s meaning. The confusion can only be compounded when considering other idiomatic uses that include the word blast. For example, jets blast off, and so do irritated people. The idiom "a blast from the past" means a memory or person has brought a strong reminder of the way things used to be. Someone who does amazingly well has blasted the competition out of the water.