"unpack" as "analyze" rather than "remove from container"
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) fig. 1596 T. Nashe Haue with you to Saffron-Walden sig. K4, The strange vntraffiqu't phrases, by him new vented and vnpackt.
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.
Searching Google Ngram for “unpack the meaning” (which I assume is a typical example), the results suggest that about 1956 was a starting point, and usage really took off from about 1986.
