I know it's currently more of a slang to attach and use it as normalish (see what I did here?) suffix, but still — was there any evolution for this? Also — maybe it had some special meaning?
The suffix -ish comes from Old English -isc and is a diminutive.
So it means the word is lessened in intensity. Normalish (while not a proper word) means a bit normal. Yellowish means the thing is a bit yellow. Smallish means something is small, but not overly so.