if you run into a curious qu- while reading archaic English, particularly if it came from Scotland, it often helps to substitute a wh- and see what you get.

This doesn't seem to work too well for quhilk, but it often does work in other instances.

Quhilk required a visit to the unabridged dictionary,which said, "for obsolete forms beginning with quh, see the corresponding forms beginning with wh."

So I looked up "whilk" and found that it is obsolete dialect for "which." At least it is when it's not a variant of "whelk."

Did the Scots used to celebrate a Whelk Day?