Origin of "It's been a slice"
Solution 1:
Meaning
The McGraw-Hill Dictionary of American Idioms and Phrasal Verbs defines it:
It's been a slice!
Sl. It's been good. Good-bye and thank you. It's been a slice! [/] It's been a slice. I hope to see you again some day.
Origin of It's been a slice!
1975
Jonathan Lighter, author of Random House Historical Dictionary of American Slang, first heard it in 1975:
I heard the ironical "It's been a slice" from a fellow graduate student in 1975. He told me he'd first heard it a year or two earlier, and that it was short for "a slice of life."
1979
The earliest I found is in newspaper Edmonton Journal (17 October 1979) in the headline and article:
Thanks Mr. Speaker, It's Been A Real Slice!
...
Consumer Affairs Minister Julian Koziak said:
"I'm pleased to introduce this bill, as a fresh piece of legislation, being an example of deregulation. By passing this bill, we will be giving a stale legislation the bun," he said.
"Thank you, Mr. Speaker, It's been a slice."
1981
The earliest I found in Google Books is a possible 1981 snippet of Art-com magazine (Contemporary Arts Press, San Francisco Bay Area):
... the page you are reminded that they, along with the city, are reeling ahead projecting new ideas, images, and ways of survival on screens that have never been projected on before.
As my friend Miss Judy would say, " This has been a slice!"
186 pages book paper
black and white photos
paper bound U.S. $ 10
ISBN 0-937122-06-8
To check the date, the book is the 1981 Young Turks by Stephen Seemayer (Los Angeles: Astro Artz, ISBN: 0-937122-06-8). So it's no earlier than this, and if a recent book review, probably from 1981 also. Here's confirmation the magazine was published under this name between 1980 and 1984.
1984
The earliest Google Books full view is Princeton Alumni Weekly (1 June 1984, Volume 84) in Class Notes for 1984 by student, Glenn N. Huber:
I'd also like to thank my roommates, Scott T. Burgess, David K . Marion, and Bill G. Manias. for a most enjoyable year—it's been a slice. guys!
1984
It was used in the film Streets of Fire, released 1 June 1984, spoken by McCoy (played by Amy Madigan). Here's subtitles from Subzin:
00:37:36 Poker's a real interesting game when you've got all the best cards.
00:37:43 Next time, right in your nuts, pal.
00:38:03 Guys, it's been a slice.
00:38:08 - Who are you? - I'm a big fan.
00:38:10 Come on. Let's go.
I can't find a clip, so here's the trailer.
1985
Another verifiable use is in the acknowledgements section of John Howard Hutchinson's doctoral thesis Use of Camphor in Natural Product Synthesis (September 1985, University of British Columbia) (pdf):
Finally, I would like to thank my research supervisor, Dr. Thomas Money, for the unending help, support and enthusiasm which he has displayed over the past four years.
Thanks Tom - it's been a slice!
1985
Another is a snippet of Something in Common (1985) by Robert Robin:
Google snippets are often misdated. To verify, here's a contemporary review confirming Cathy is a mother with "two sons--Andy, 14, and Judd, 11". Here we can see it used by the teenager to his younger brother before leaving.
The family live in the village of Glencoe, Illinois and the father commutes to Chicago, some 25 miles away. The author is also from Chicago.
Origin of It's been a real slice!
The 1979 Edmonton Journal used both It's been a slice! and It's been a real slice!. Perhaps the "real slice" version came earlier.
1980?
Armed Forces Journal International, from I think 1980, has this (not shown in the snippet, just search results):
I've had it — life's too short to fight an uphill battle for Commanders and Staffs who won't listen (Remember CORONA ACE?) or don't believe, or maybe don't even care. So thanks for the memories, it's been a real slice of life . . . but ...
So did the phrase come from "a real slice of life"?
1984?
A snippet in the Virginia PTA Bulletin (I think from shortly after March 1984, see bottom right of snippet) shows it had a negative meaning:
Students who have had a terrible day often say, "It's been a real slice;" if they've been treated unfairly, they say they've been "munched on" or "dogged." When someone plays a trick on them, they've been "schroedered."
Summary
These don't help explain what the slice actually was, but it helps place it in time and space. My own guess is it's a playfully ambiguous suggestion the experience was a "slice of heaven"; or was it a "slice of hell"? Or perhaps, as one example shows, from "a real slice of life", a full experience.