Green’s Dictionary of Slang

dumcow v.

[alteration of Hind. धमकाना (dhamkānā) to berate, to give a drubbing to (ultimately from धम (dham) a loud sound), based on dumb, mute + cow, to cause to feel disheartened]

(orig. Anglo-Ind.) to bully, to browbeat.

[US]Masonic Mirror (Boston) 29 Apr. 140/2: Defendant’s Counsel, Dolcheed Cowmuddy, Esq. was quite taken back by it, and Counsellor Moorgee, who never loses an opportunity for cracking a joke, said his brother Cowmuddy appeared rather Dum cowed.
[Ind]F.J. Bellew ‘Memoirs of a Griffin’ in Asiatic Jrnl & Mthly Register May 52: However, I don’t think we have gained much by his budlee (successor), our new kummadan (commandant) a regular bahadur (great person), who [...] dumcows (bullies) the native officers, and gallees (abuses) the Jacks (sepoys).
[Ind]J.H. Stocqueler Oriental Interpreter 79/2: DUMCOW, Hindostanee. Verb, to bully.
[Ind]Kipling Wee Willie Winkie 63: Their one excuse is that they came again and did their best to finish the job in style. But for a time all their world knows that they were openly beaten, whipped, dumb-cowed, shaking and afraid.
[Ind]M. Macmillan Globe Trotter in India 88: The Anglo-Indian derivative is spelt ‘dumbcow,’ so as to give both syllables an English meaning, and raise in the mind the idea of cowing a person and rendering him dumb, or of making him as dumb as a cow.
K.R. & M. Chatterjee [trans] B. Banerji Pather Panchali (1990) 125: Opu was shy and silent in any case and this treatment dumb-cowed him completely.