If you unpack an idea or problem, you analyse it and consider it in detail.
(Collins Dictionary)
I couldn’t find information about the origin and usage of the above term, the only possible hint might come from Google Books which shows a clear spike in usage of “unpack” from the late 90’s early 2000’s.
Is the figurative, psychological usage of unpack so recent? Or is the spike in usage due to other factors?
This is actually quite an old use, with the OED's first citation from 1596.
The OED gives the meaning "1 b. transitive. figurative and in figurative contexts; esp. to reveal, disclose, or exhibit (a thought, feeling, quality, etc.)."
The first example is from Thomas Nashe's Have With You To Saffron Walden (1596): in the original spelling "The strange vntraffiqu't phrases, by him new vented and vnpackt." A bit more context is given here although you may be forgiven if the meaning is still a bit obscure:
When I record (as I do often) the strange untrafficked phrases by him new vented and unpacked, as of incendiary for fire, an illuminary for a candle and lantern, an indument for a cloak, an underfoot abject for a shoe or a boot, then I am ready (with Erasmus) to cry, Sancte Socrates, or (with Aristotle), Ens entium miserere mei, What an ingeny is here!
A very modern-sounding example is from 1755 in Tobias Smollett's translation of Cervantes' Don Quixote II. iii. vi. 236:
Your ladyship may ... unpack and recount your griefs, that all of us may understand the nature of your misfortune.
In this example, a woman is invited to explain and elaborate upon the sources of her sadness.
It's an extension of the literal sense, to take something out of a bag to use or examine it, applied to abstract ideas.
The metaphorical usage of unpack to unload emotions is quite old, certainly older than modern psychology or the phrase emotional baggage. The specific application to analysis emerges no later than the early 20th century.
The Oxford English Dictionary, "unpack, v.", has a few relevant entries that help approximate when unpack attains these figurative meanings. The first, def. 2.a., involves confiding one's emotions:
transitive. figurative and in figurative contexts, esp. with reference to the releasing of one's emotions. Also reflexive: to unburden oneself emotionally.
1604 W. Shakespeare Hamlet ii. ii. 588 This is most braue, That I..Must like a whore vnpacke my hart with words. [Note: this is the only use of unpack in the Shakespeare corpus, based on an Open Source Shakespeare search.]
1655 T. Harvey tr. G. B. Spagnoli Bucolicks ii. 11 Men do the like; each doth himself unpack, And casts the burthen on anothers back.
[...]
2001 Community Care 13 Dec. 88/2 (advt.) Techniques such as play therapy, art and drama are used to help the child ‘unpack the suitcase’ of emotional baggage.
So the idea here is not necessarily analysis but rather confiding in another. If the emotions are packed in, telling them in words unpacks them. I can find examples of this word then being used to pertain to other sorts of telling or disclosing, as this preface to A Third Defence of the Cause of Peace (Richard Baxter, 1681) does in response to a "Mr. Cheney":
His Book consisteth partly of a handsome considera∣ble discourse for Prelacie, and other Church-Offi∣ces of Humane Invention; and partly of a new & singular Doctrine about Church-Forms; & part∣ly in a critical discharge of his fancy, and unpack∣ing his preparations against the Independant Co∣venant, and Church-Form;
The critical discharge of his fancy relates to Mr. Cheney's imagination being fired off. Baxter switches metaphors to then describe unpacking his preparations, or disclosing his plans or acts.
A later usage from Arthur Murphy's play Know Your Own Mind (1778) involves conveying what other people have said:
Sir Harry: By the shade of Rablais, he is the most entertaining creature! He has played off such a firework of wit. I'll tell you what he said this moment.
Byg: No, Sir, no; if you are a pedlar in smart sayings and brisk repartees, we don't desire you unpack for us.
A more analytical usage emerges in the 20th century, documented in def. 2.c.:
transitive. Originally Philosophy. To analyse (an issue or concept) in great detail, typically with the aim of uncovering its underlying assumptions or hidden implications. Later more generally: to analyse (a work, etc.) in order to interpret or understand it.
1906 G. S. Fullerton Introd. Philos. xii. 179 Thus, when we say: Man is a rational animal, we may merely be defining the word ‘man’—unpacking it, so to speak.
1953 R. J. Spilsbury in Mind 62 353 The meaning of such statements can be unpacked in a series of hypothetical propositions.
So to unpack in these contexts is no longer mere telling but analysis: to discover the underlying assumptions and implications of something. Modern figurative usage can involve disclosure, analysis, or possibly both.
Building on the various comments, I ran Ngrams against unpack_INF ideas. (The same search with "idea" finds nothing) Unpack ideas has a small "hump" in the mid 1950s and there is some mention of both between 1975 and 1995, then they both take off. So it looks as if the phrase started to be used significantly around 1995.
However this is for all books, not just those on psychology. Many of the books listed seem to be about teaching and other subjects instead. You would have to plough your way through several pages of Google search results to find the earliest reference in a work you could classify as psychology
