Green’s Dictionary of Slang

borrow and beg n.

an egg.

1910		Eve. Teleg. (Dundee) 3 Aug. 4/1: He then may partake of two hard-boiled 'borrows' (borrow and beg' - egg).
1960		J. Franklyn Dict. of Rhy. Sl. 41/1: borrow and beg An egg Late 19C. The term [...] enjoyed a fresh lease of life during the second world war food-rationing period; now, again sinking into oblivion.
1969		S.T. Kendall Up the Frog.
1981		P. Wright Cockney Dialect and Sl. 103: borrow an’ beg ‘egg’.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.