Is there such phrase in English as "slick in the eye" to have the meaning as "directly to the eye"?
"A glass bead hit him right in the eye." or
"A glass bead hit him slick in the eye."
Using “slick” in that context would confuse most English speakers, and I suspect few if any would guess that the intended meaning was “directly”.
Slang uses of “slick” could, at a stretch, make it work, but the meaning would be something like “cleanly” or “efficiently”, or even “impressively”. However, that is, as I say, a stretch.
So, no, there is no such English phrase as, “slick in the eye”.
The closest I can come up with is "smack-dab"
A glass bead hit him smack-dab in the eye
I have never heard "slick in the eye"