from Old English dæl "part, share, quantity, amount," from Proto-Germanic *dailaz (cf. Old Norse deild, Old Frisian del, Dutch deel, Old High German and German teil, Gothic dails "part, share"), from PIE *dail- "to divide" (cf. Old Church Slavonic delu "part," Lithuanian dalis).
Business sense of "transaction, bargain" is 1837, originally slang. Meaning "an amount" is from 1560s. New Deal is from F.D. Roosevelt speech of July 1932. Big deal is 1928; ironic use first recorded 1951 in "Catcher in the Rye." Deal breaker is attested by 1975.