"a part or portion," Middle English del, from from Old English dæl "a part of a whole, a share;" with qualification (great, etc.), "an extent, degree, quantity, amount," from Proto-Germanic *dailaz (source also of Old Norse deild, Old Frisian del "part; juridical district," Dutch deel, Old High German and German teil, Gothic dails "part, share, portion"), from PIE *dail- "to divide" (source also of Old Church Slavonic delu, Lithuanian dalis "part"), perhaps a Northern Indo-European extended form of the root *da- "to divide," or perhaps a substratum word.
Formerly used in many senses now taken by part. Meaning "a share (of something), one's allotted portion" is from c. 1200. Business sense of "transaction, bargain" is 1837, originally slang, from the older sense of "arrangement among a number of persons for mutual advantage." In American history, New Deal is from Franklin D. Roosevelt's speech of July 2, 1932 (the phrase itself is by 1834). Big deal is from 1928 as "important transaction;" ironic use first recorded 1951 in "Catcher in the Rye." Deal-breaker is attested by 1975.