Green’s Dictionary of Slang

1. (US) a mild excl., a euph. for God!

1863		in Journal Illinois State Hist. Society LVI (1963) 161: O cow, the drums [...] ain’t worth much [HDAS].
1930		L. Sage Last Rustler 129: Cow hell! I was duded up as far as hands, face and feet went.

2. (UK, mainly northern) a euph. for fuck! excl. (1)

1977

(con. 1940–50s) Nicholson & Smith Spend, Spend, Spend (1978) 18: Oh, cow him.