Sanskrit

Adverb

दुस् • ‎(dus)

1. a prefix to nouns and rarely to verbs or adverbs (Pāṇ. 2-1, 6 ; 2, 18 Vārtt. 2 Pat. ; iii, 3, 126, etc.) implying evil, bad, difficult, hard
2. badly, hardly ; slight, inferior, etc. (opp. to सु), often = Engl. in- or उन्-.

Usage notes

This prefix

• becomes दुर् before vowels and soft consonants;
• becomes दू before र् and sometimes before द्, ध्, न्, which become ड्, ढ्, ण्;
• remains unchanged before त्, थ् (in older language however ष्ट्, ष्ठ् result);
• becomes दुष्, rarely दुः, before क्, ख् ; प्, फ्;
• becomes दुश् before च्, छ्;
• becomes दुः, rarely दुश्, दुष्, दुस्, before श्, ष्, स्.

References

• Monier William's Sanskrit-English Dictionary, 2nd Ed. 1899.