Grimms' glossed German einschlägig thus:
spectans, pertinens ad aliquid, bezüglich
I was looking to understand what einschlägig means, which is only used in fixed expressions, and I'm afraid I didn't get very far, but if you have to ask (if the gloss isn't enlightening):
That is however not quite obvious from the older usage example in Grimm, nor from the glosses. Further elaboration is besides the point, though, cp. perhaps Grimms' entry at bezüglich.
From en.wiktionary I can glean that aliquid is an inflected form of aliquis "something" ("other" + "what")
Now it dawns on me that any sense of sameness might be conferred by the inflected construction with ad, as if "of one thing", "pertaining to quite something else."
Is that it?
What a shame, I was gonna run through a couple etymology notes about the same. I'll repost under the etymology tag pending confirmation.