to enjoy doing something very much.

Sky watchers are having a blast keeping track of the comet.

They had a blast poking around the farmers’ market looking at all the food.

• be in the saddle
• be in the same ballpark
• be in the same boat
• be in the shit
• be in the soup
• be in the spotlight
• be in the thick of
• be in the thick of something
• be in the throes of
• be in the throes of doing something
• be in the throes of something
• be in the wind
• be in the wrong
• be in too deep
• be in touch
• be in train
• be in two minds
• be in your blood
• be in your cups
• be in your element
• be in your face
• be joined at the hip
• be jolly hockey sticks
• be just a pretty face
• be just the job