The answer is 1596. That is the first example in the OED of a figurative meaning of 'unpack' - see below.
The OED does include a specific meaning of 'unpack' as relates to computing. Its first reference is 1954.
OED meaning 2a. (part-of)
1596 T. Nashe Haue with you to Saffron-Walden sig. K4, The strange vntraffiqu't phrases, by him new vented and vnpackt.
1821 C. Lamb Imperf. Sympathies in Elia 1st Ser., He brings his total wealth into company, and gravely unpacks it.
1841 H. Miller Old Red Sandstone vi. 107 The strata..have been unpacked and arranged by the uptilting agent.
1966 Philosophy XLI. 30 If we unpack what is meant by ‘positive morality’ we come down to the opinions of individual people.
1979 A. R. Peacocke Creation & World of Sci. i. 39 The theological enterprise has always involved much unpacking and elaborating of this image.
OED meaning 5.
- Computing. To convert (an item of stored data) into two or more separate items; to retrieve data from (a record). Cf. pack v.1 4d.
1954 Computers & Automation Dec. 22/2 Unpack, to separate packed items of information each into a separate machine word.
1959 M. H. Wrubel Primer of Programming for Digital Computers viii. 189 If the data consist of only a few significant digits, two or more numbers can be ‘packed’ into a single 10-digit word. They will be transferred from the card to the machine as a 10-digit word, which must subsequently be ‘unpacked’ by an appropriate program.
1960 R. H. Gregory & R. L. Van Horn Automatic Data-processing Syst. iii. 104 The 13 instructions given above..are required to unpack each meter-reading card and set up the quantity for bill calculation.
1972 Computer Jrnl. 15 199/1 A stream..might be formed by one stream function which unpacks words into bytes.