Dining Room Lumber (toothpick)

“Dining room lumber” was lunch counter slang for a toothpick—an item made of wood (“lumber”). “Tooth pick: Dining room lumber” was printed in the article “Lunch Room Jargon” in the Middletown (NY) Times Herald on September 9, 1936.
 
Lunch room slang became mostly historical by the 1950s and 1960s.
 
 
9 September 1936, Middletown (NY) Times Herald, “Orange Blossoms: Lunch Room Jargon,” pg. 4, col. 4:
Tooth pick: Dining room lumber.
     
Google Books
American Speech
1936
Pg. 43:
DINING ROOM LUMBER. Toothpick.
     
Google Books
Punch
1939
Pg. 300:
Such a man usually turns out to be the type who thinks nothing of writing an epilogue to his repast by making public use of
Dining-room lumber. A toothpick.
 
26 February 1954, La Crosse (WI) Tribune, “Weird Words Uncovered By UW Expert In State Study,” pg. 8, cols. 1-2:
MADISON (Special)—Some people in Wisconsin can eat an “old maid” and then clean their teeth with “dining room lumber” without being reported as dangerously eccentric by their neighbors.
(...)
Translated, an “old maid” is the last piece of food on the plate; “dining room lumber” is a toothpick; the “tin rooster” is an alarm clock; “cackleberries” are eggs; and “collywobbles” are the rolls of dust which gather under beds or other furniture.
 
Google Books
Hash House Lingo:
The Slang of Soda Jerks, Short-Order Cooks, Bartenders, Waitresses, Carhops, and Other Denizens of Yesterday’s Roadside
By Jack Smiley
Introduction by Paul Dickson
Mineola, NY: Dover Publications, Inc.
2012 (Originally published in 1941)
Pg. 50:
Dining room lumber—toothpicks