a British soldier's informal and (usually) affectionate term for "Britain" or "England," popularized in World War I but attested by 1896 in India, an alteration of Hindi bilayut, billait, which is from Arabic wilayat "a kingdom, a province," which apparently was used by various peoples in South Asia in reference to their distant homelands, and in India came to be used for "Europe" generally.
WHEN Johnnie comes frae Blighty you can see it in his face,
For its jist a long's an elder's whan he's gaun tae say a grace;
He lies spare upon his charpoy, an' he never says a word,
Thats a specimen o' a'Johnnie whan he joins the Ninety-Third.
["L. Ferguson," first verse of Johnnie frae 'Blighty,'" in The Thin Red Line, The Regimental Paper of the 2nd Batt. (Princess Louise's) Argyll & Sutherland Highlanders, Nowshera, Peshawar, India, September 1896]