The word 'pizza' started to become popular in English somewhere around the mid-1930, but was not at all well-established until the 1960s. Before the modern term became popular, the primary contender for naming this dish was Italian rarebit.
Rarebit is a corruption of rabbit... which is still rather confusing. The story goes that in the late 1700s the Welsh working class either could not afford, or were generally discouraged from hunting, rabbits, and so a 'Welsh rabbit' was a piece of toast topped with well-spiced melted cheese sauce. This terminology grabbed the public's imagination, and soon there were various terms for related concoctions, including English rabbit (in which the toast was soaked in red wine), Scottish rabbit (in which the whole deal was fried, naturally), and Irish rabbit (made with stout).
And then there was the Italian rabbit, which was really fancy. A 1915 recipe called for toast topped with a well-mixed mess of spicy tomato sauce, sautéed onions and peppers, cheese, and eggs. It is noteworthy that, like Welsh Rabbit, mustard was used in the original recipe.
Well, over time 'rabbit' morphed to 'rarebit', and to this day it is mostly a matter of personal preference whether you call your cheese-sauce-and-toast dish a rabbit or a rarebit. In the case of Italian rarebit, however, history appears to prefer the corrupted form, perhaps because now that no one knows what Italian rarebit is, 'Italian rabbit' would be doubly confusing. More importantly, however, history prefers Italian rarebit to be cooked with a crust like a pie rather than constructed on toast, and thus the English eventually stole the Italian 'pizza', meaning 'pie' or 'tart', and never looked back.