rud

The root rud comes from Latin rudis, an adjective for anything still in its rough, unworked state. A rudis block of marble was uncarved; a rudis soldier was untrained; a rudis field was uncultivated. The single thread running through all of these is 'not yet shaped by skill or effort' — raw material before the craftsman, the teacher, or the manners of society have touched it.

English inherited this idea along two channels. Directly from Latin came rude (originally 'unpolished, uneducated,' only later narrowing to 'bad-mannered'), crude (still raw — crude oil is unrefined petroleum, a crude drawing is roughly done), and rudiment / rudimentary (the first rough beginnings of a skill or organ). The standout is erudite: e- ('out of') + rud = literally 'out of the rough.' An erudite person has been polished out of their original rawness through years of learning. The word quietly contains its own opposite.

A second cluster — rural, rustic, rurality — comes from the neighboring Latin rus / ruris ('the countryside, open land'). Romans saw country life as plainer and rougher than the polished city, so the 'rough, unrefined' overtone carries over: rustic furniture is charmingly unfinished, a rustic manner is unsophisticated.

Finally, the homely Germanic word raw is a true cognate: Latin rudis and English raw both descend from the same Proto-Indo-European root for 'raw, bloody.' That is why raw food, raw skin, and a raw recruit all share the family feeling of 'unprocessed.' Brute and cruel sit at the edges: both carry the 'savage, unrefined' sense, with cruel reaching English through Latin crudelis (from the same crud- 'raw, bloody' family). The pattern across the whole family: start with something raw and unformed, then either leave it rough (crude, rustic) or polish it out (erudite).