What is the literal and figurative meaning of "brief-case" in the following excerpt from the chapter Seven of The Great Gatsby?
Thirty—the promise of a decade of loneliness, a thinning list of single men to know, a thinning brief-case of enthusiasm, thinning hair.
After wasting an exhausting afternoon in the torrid heat of a summer day in New York City amid the equally heated exchanges of passion among Daisy, Gatsby, and Tom, Nick suddenly realizes that this day is his birthday:
I was thirty. Before me stretched the portentous menacing road of a new decade.
This "formidable stroke of thirty" that presages what should be fully adult maturity and establishment of a career and a family promises Nick only
...a decade of loneliness, a thinning list of single men to know, a thinning brief-case of enthusiasm, thinning hair.
The metaphor of "a thinning brief-case of enthusiasm" means that Nick's youthful attitude and looks will wane and the future may hold less for him if he wastes much more time with careless drinking and frivolous company. For, dreams can only be carried for so long, or they become forgotten and wasted.
This passage from the novel, along with Daisy's cynically existential question
“What’ll we do with ourselves this afternoon and the day after that, and the next thirty years?”
is suggestive of T.S. Eliot's "The Wasteland," a poem that greatly influenced Fitzgerald. The portrayal of the Buchanans and their passionate interaction with Gatsby is trivialized by Daisy's "lost voice across the room" asking her husband to stop his exposure of Gatsby's history. Then, Tom's dismissal of Gatsby as insignificant further taints his dream of capturing Daisy as it is "snapped out, made accidental"--wasted. And, as a witness and part of all this, Nick Carraway expresses his sense of time wasted by reflecting upon his thirtieth birthday and its metaphoric "thinning brief-case."