the (in) thing
The newest, most recent, or most current trend, fashion, or style. Deep V-necks are becoming the in thing for wedding dresses lately. Smooth, edgeless bodies are the thing for smartphone designs these days.
the in thing
That which is currently (and usually temporarily) fashionable or trendy. Haven't you heard? Wearing knee-high socks is the in thing now!
the in thing (to do)
the fashionable thing to do. Eating low-fat food is the in thing to do. Bob is very old-fashioned. He never does the in thing.
in thing, the
see under the thing.