In general, there is a prohibition to adopt practices from other cultures (See, e.g., Maimonides' Laws of Idolatry 11; see also here) if there is no clear, reasonable, non-idolatrous justification for the practice (e.g. knocking on wood) for fear that it has its roots indeed from the rituals of idolatry. I believe this is the case regardless of whether or not the actual origins are known, provided there isn't a halachically-significant reasonable justification (e.g. flossing[?]).
What about specific aesthetic patterns or symbols that have been in use since antiquity (e.g. celtic knots)? What if the common practice in the observant Jewish world has already been to incorporate these patterns (e.g. braided loaves)? What if there is significant evidence that the origins do indeed have pagan ritualistic undertones?