“Taco short of a combination plate” is another way of saying that a person is “not all there” or “not playing with a full deck” (called “full deckisms”). The expression dates to at least 1986 and was popularized in 1992, when comedian Robin Williams said this about Vice President Dan Quayle.
 
The Mexican combination plate is claimed to have been invented by Dallas restaurateur Miguel Martinez and originally contained a tamale, an enchilada, beans and rice (no tacos).
   
     
Cliché Site
Cliché: One taco short of a combination plate
Explanation:  1. A derogatory expression used to describe someone who is not very smart or who is dumb.
Country:  United States
 
Google Books
Cassell’s Dictionary of Slang
by Jonathon Green
Sterling Publishing Company, Inc.
2006
Pg. 1483:
two tacos short of a combination plate phr. [1980s](US) not very intelligent, slightly eccentric, odd. [var. on NOT ALL THERE phr.] 
   
21 July 1986, Los Angeles (CA) Times, “Glad to be back in ‘Amazing Smaze’” by Jack Smith, section 5, pg. 1:
As a phrase floating around this newsroom has it, we’d think he was “one taco short of a full combination plate.”
   
30 July 1986, Boston (MA) Globe, “Kennedy has harsh words for Rehnquist rights record”:
...an extremist…a trampler of the poor, sexist, an unwell man and a crazed law clerk who is about two tacos short of a combination platter.
 
15 August 1986, Philadelphia (PA) Inquirer, pg. B2:
...expression several times that indicates that a person isn’t wrapped too tight. In Texas they say that such a person is “two tacos short of a complete meal.”
 
29 May 1988, Los Angeles (CA) Times, “Living a better Life From the Catalogue” by Alice Kahn, section 6, pg. 1:
I’m only three credits short of a BS and one taco short of a full plate.
       
23 February 1989, Washington (DC) Post, “Two Tacos Short of a Combination Plate,” pg. A18:
As is their wont, the ever puckish pair of Rowland Evans and Robert Novak have penned a column that misses just a few key points—especially in their discussion of me {"John Tower: Victim of Rumor ..." op-ed, Feb. 8}. As we say in Wyoming, their reporting is about two tacos short of a combination plate.
   
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From: .(JavaScript must be enabled to view this email address)
Date: 19 Sep 90 00:18:15 GMT
Local: Tues, Sep 18 1990 7:18 pm
Subject: More Ways to say Your Crazy (was Re: Quotes2)
   
Many of these come From Rescue Rangers, which has even more ways than Foghorn Legorn.
 
’...about a quart low’—Foghorn Leghorn, et. al.
‘Her elevator doesn’t quite go to the top floor’—Monty
‘A few sticks short of a bundle’—Monty
‘a few snowflakes short of a blizzard’—Monty
‘A Taco Short of a combination platter.’—Muldoon
 
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From: .(JavaScript must be enabled to view this email address) (Mike Hammar)
Date: 18 Mar 91 22:24:02 GMT
Local: Mon, Mar 18 1991 5:24 pm
Subject: Re: Cliches (was Re: Schoolhouse Rock)
 
There’s always my favorite: “Two tacos short on the combo plate.”
 
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From: .(JavaScript must be enabled to view this email address) (BACS Data Communications Group)
Date: 15 Apr 91 02:22:32 GMT
Local: Sun, Apr 14 1991 9:22 pm
Subject: Fulldeckisms
 
He’s two tacos short of a combination plate.
 
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From: .(JavaScript must be enabled to view this email address) (Greg Cagle)
Date: Mon, 20 Jan 1992 16:58:31 GMT
Local: Mon, Jan 20 1992 11:58 am
Subject: Re: Whale Blow Up
 
“That guy’s one taco short of a combination plate!”
 
8 February 1992, Chicago (IL) Daily Herald, section 1, pg. 8, col. 1:
This strategy pays off only when the smoker gets the impression I am a taco short of a combo platter and takes steps on his own to put distance between us.
 
27 May 1992, Indiana (PA) Gazette, pg. 10, cols. 3-4:
The reaction to Quayle’s remarks fell loosely into three camps: those who thought it reinforced the perception, as comedian Robin Williams put it on Carson last week, that Quayle “is one taco short of a combination plate”; those who thought he had a valid point about the sanctity of the American family; and those who thought he was dumb like a fox.
 
1 January 1997, New Mexican (Santa Fe, NM), pg. A3, col. 4:
Coulter Evans, a bartender at the Catamount Bar, 125 East Water St., said there are definite signs when someone becomes two tacos short of a combination plate.
 
12 May 1998, The Reporter (Emmetsburg, Iowa), pg. 4, col. 3:
Suggesting that they were at least two tacos short of a combination plate, Burton and Bossie simply dropped those sentences that cast Hillary Rodham Clinton in a more positive light out of the middle of paragraphs.