On Stenk is somewhat similar to its predecessor “on fleek” meaning of high fashion, perfection and looking fly. Other words are available.
On Stenk is used, most commonly, as a way to describe someone’s physical appearance and/or outfit when they are fully rocking a top quality look and you can’t just not comment.
For example:
Jon: were you at the wedding of the year?
Hannah: James and Claire? Yes.
Helen: Claire was fully on stenk!
Keith: RETWEET!
Oh no the stonks market is down this stinks
worker: hey we are booming with company!!
worker 2: yeh
stonks man: oh noes stonks market is crashing
worker: can we call it stenks now