If 'ish lto inm' stands for 'neat and tidy' ; 'qpr inm sen' stands for 'small but neat' and 'hsm sen rso' stands for 'good but erratic', what would 'but' stand for ?

Answer: C. sen

In the second and third statements, the common code word is 'sen' and the common word is 'but'.
So, 'sen' means 'but'.