There is something very odd with κυριακός My dictionary Babiniótis does not give κυριακός at all, it only gives the adj. κυριακάτικος. But in the ethymology it says:
Κυριακή a substantivation of the ADJECTIVE κυριακός, and
κύριος/ mister, sir, lord - ancient κύρος - I.E. keu- (to swell/ expand), also cumulus and cumulate come from this as well as the Gr. wave = κύμα
Babiniótis also gives Pater Noster/ Pάτερ Ημών as Κυριακή προσευχή , prayer of Sunday, where κυριακή is an adjective;;;???
It was an adjective in Ancient Greek, but it's really "underused" in Modern Greek, and actually used in "fossile" phrases such as Κυριακή προσευχή (unless you write a poem or something!). Κυριακάτικος is the "related to Sunday", not "related to Lord". There is also the name Κυριάκος/Κυριακή which is actually that adjective as a name.