Green’s Dictionary of Slang

sonofa n.

also sonova

abbr. of sonofabitch n.

[US]Champion Jack Dupree & Mickey Baker ‘My Black and White Dog’ [lyrics, spoken] He didn’t say a word, and as soon as I opened the door the sonofa jumped at me you know.
[US]H. Rawson Dict. of Invective (1991) 366: The large number of euphemisms that were devised for it (such as S.O.B., from ca. 1850, so-and-so, sonofa, son of a b., son of a sea cook, son of a gun).