What is the literal and metaphorical meaning of "brief-case" in the following passage from The Great Gatsby:
"It was seven o’clock when we got into the coupé with him and started for Long Island. Tom talked incessantly, exulting and laughing, but his voice was as remote from Jordan and me as the foreign clamor on the sidewalk or the tumult of the elevated overhead. Human sympathy has its limits and we were content to let all their tragic arguments fade with the city lights behind. Thirty—the promise of a decade of loneliness, a thinning list of single men to know, a thinning brief-case of enthusiasm, thinning hair.
Extracted from Chapter Seven, the climactic chapter of The Great Gatsby, in which Gatsby makes his claim for Daisy, this passage is an introspection of Nick in the wake of his realization of the shallowness of the lives the Buchanans and Jordan and other East Eggers. When Daisy, seduced by Gatsby's wealth and opulent mansion, so easily changes her mind when she learns from Tom that his money is ill-gotten and he will never be accepted by the socially elite, she retreats to her world with Tom, leaving Gatsby a "dead dream" to struggle to touch and catch "that lost voice across the room."
After the conflict between Tom and Gatsby and that within Daisy, Nick observes as they depart from the Plaza Hotel,
They were gone, without a word, snapped out, made accidental, isolated like ghosts even from our pity.
This observation explains why Tom's voice seems remote to Nick, for Fitzgerald's narrator realizes the hollowness of the lives of these residents of East Egg. This realization causes Nick to reflect upon his own life that he has been frittering away lately. Looking forward to his thirties, he sees no promise of fulfillment, either, only "a decade of loneliness" as men that he knows will have married and settled down to their professions and moved away from him. After having enjoyed the irresponsibility and recklessness of his twenties, frittering away evenings with people who mean little to him, drinking, and then returning home, Nick takes stock of what the future portends: "a thinning list of single men...a thinning brief-case of loneliness." Here the metaphor of a brief-case compares his life and what has been important in it--people hold important papers and documents in brief-cases--as shallow and empty, a place of "loneliness."