Moggy
Alternative forms
Origin
In sense “mongrel catâ€, 1911, of Unknown origin, possibly Cockney. Possibly derived from maggie, margie or mog, all short forms of the female name Margaret.
Original sense, early 19th century, is a term of affection for a calf or cow, which may have been transfered to cats under urbanization. Later 19th century meaning of “untidy woman, slatternâ€. Alternatively, in Wigan, moggy traditionally applied to mice, not cats, and a cat was hence a moggy catcher, which may have been abbreviated to moggy.
OED
Apparently not an abbreviation of similar-seeming mongrel, though perhaps from similar Old English/Proto-Germanic source; see mongrel for details.