...when I asked my coworker to explain...why is the code shit, I couldn't get a straight answer.

Maybe because there are only so many straight answers and we get tired of repeating them so often...

Class A: It's too difficult for the next programmer to understand this

  - poor (or horrible) variable naming
  - poor (or horrible) function naming
  - repeated repeated repeated repeated code
  - comments don't match code
  - comments needed but not there
  - comments needed by unintelligible
  - unbalanced structure
  - poor (or horrible) choices for iterations
  - poor (or horrible) choices for conditionals
  - poor (or horrible) choices for data design/storage
  - poorly maintained by Programmers #2 thru x, who didn't understand it
  - etc., etc., etc.

Class B: Poor machine performance, not by 10%, but by 10000%, because of extremely poor design and development choices.

Class C: It's broken. Insufficiently tested & broken in production.