The word is an odd mix of Latin (ultra) and Greek (crepidas). It means to go beyound your step. Basically it means to opine about a subject that is beyond your expertise or that you have no business talking about.
Here is what "World Wide Words" has to say about how we got this interesting word:
Ultracrepidarian comes from a classical allusion. The Latin writer Pliny recorded that Apelles, the famous Greek painter who was a contemporary of Alexander the Great, would put his pictures where the public could see them and then stand out of sight so he could listen to their comments.
A shoemaker once faulted the painter for a sandal with one loop too few, which Apelles corrected. The shoemaker, emboldened by this acceptance of his views, then criticised the subject’s leg. To this Apelles is reported as replying (no doubt with expletives deleted) that the shoemaker should not judge beyond his sandals, in other words that critics should only comment on matters they know something about. In modern English, we might say “the cobbler should stick to his last”, a proverb that comes from the same incident. (A last is a shoemaker’s pattern, ultimately from a Germanic root meaning to follow a track, hence footstep.)
What Pliny actually wrote was ne supra crepidam judicaret, where crepidam is a sandal or the sole of a shoe, but the idea has been expressed in several ways in Latin tags, such as Ne sutor ultra crepidam sutor means “cobbler”, a word still known in Scotland in the spelling souter). The best-known version is the abbreviated tag ultra crepidam, “beyond the sole”, from which Hazlitt formed ultracrepidarian.
Crepidam derives from Greek krepis, a shoe; it has no link with words like decrepit or crepitation (which are from Latin crepare, to creak, rattle, or make a noise) or crepuscular (from the Latin word for twilight), though crepidarian is a very rare adjective meaning “pertaining to a shoemaker”.