en.wikipedia.org/wiki/Wiktionary:ish
Check out this link which defines what “ish” means.
As an adverb it means 1) somewhat as in “How was the film?” “It was good, ish.” meaning it was somewhat good.
or 2) reasonably, fairly as in “The exam went well, ish.”
meaning pretty well although not as good as I wanted
(in this context the “ish” is better than number 1)
or 3) about, approximately (wikipedia does not give an example, this one is mine) “childish” vs. “child” If you are acting childish your are approximating a child. (This last “ish” is even closer than the other two.)
Synonyms given are: around, near, nearly, almost, about, loosely, roughly, close to
I think this really sheds light on “Yeshivish”. Its a very hard term to peg hole and people take it to mean different things because as we have seen, “ish” ranges from o.k to close to.
As can be seen in the comments, some people claim that “yeshivish” is just a code word for a style and attitude, however the grade of frumkeit lies in yiras shamayim, others take “yeshivish” as the given indicator for everything.
The reason for the confusion is that “ish” can mean similar but not the same. And in our context it would describe someone who acts,looks,speaks… like a yeshiva product but is not the real carbon copy. He may lack in some areas, which obviously all people do, but being that “ish” is not exact, “yeshivish” is not the yardstick to judge with.