Can someone explain the derivation of this phrase ”your ass is grass”? Does it have something to do with a mule & a putting green? It is used in threats such as, and I quote, “but don’t try doing it on my car because if I catch you, your ass is grass!”
Y’all

the full phrase, i believe, is "your ass is grass, and i'm the lawnmower"

The last expression is the one I am familiar with. First heard it in Navy boot camp. "Your ass is grass, and Battalion is the lawn mower."

Y’all, I first heard it used when I was in the Army in 1960s. Never heard the lawnmower part and thought it to mean ‘your ass will be in shreds as is grass’ with the rhyme being the main thing – but that was only my guess.
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DICTIONARY OF AMERICAN SLANG by Chapman

(ONE’S) ASS IS GRASS, sentence from the 1940s: The subject is in trouble; one will be ruined, undone, etc. <Give me a title, in short, or your ass is grass.—Boston Magazine
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Ken G – June 18, 2003

What about the phrase cover your ass? I could not find anything on this and I know it is used often.
Ahmed
BC Canada

Ahmed, This phrase was originally military and arose in the 1950s [probably during the Korean War]. It’s acronym, also widely used, is C.Y.A. This expression saw duty in Viet Nam and is commonly used in government, business, and elsewhere. Some more discrete folks in other lands (see below) use the bowdlerized version “cover one’s back.”
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Cover means provide protection and in almost any old Western B-movie someone had to say something to the effect, “Cover me Zeke. I’m goin’ in.”

COVER transitive verb, Military: to protect and guard from attack (a soldier, force, or military position) during an expected period of ground combat by taking a position from which hostile troops can be fired upon. <units covering the retreat of the main army> <ships covering approaches to the harbor> <. . . just lawyers’ endless loopholes and cover-your-ass clauses> (Random House and Merriam-Webster’s Unabridged Dictionaries)
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COVER ONE’S ASS/TAIL, slang (vulgar) [mid 1900s to present]. 1) verb phrase: to take measures that will prevent one from suffering blame, loss, harm, etc.; to protect oneself; to act in advance of some hazard to protect one’s interest; to provide or arrange for exculpation; devise excuses and alibis. Cf. C.Y.A. <Some call it ‘risk management,’ others ‘covering your ass’—Toronto Life> <The FBI may have to let you be destroyed to cover its own ass—Nat Hentoff> <CYA, you know, that old French expression that means making sure that when historians write about it all it wont be seen as happening on your shift—Washington Post>. 2) adjective phrase: <writing long cover-your-ass memos—Village Voice>. (Random House Unabridged Dictionary and Slang and Euphemism by Spears, Chapman’s Dictionary of American Slang)

COVER YOUR ASS/C.Y.A. or CYA, phrase [1950s and still in use] (originally U.S. military): look after yourself before worrying about anyone else, be it colleagues, customers, the larger world, whatever; the basic admonition to anyone, at any level, working in government or a large corporation. (Cassell’s Dictionary of Slang)

COVER ONE’S BACK (or North American ‘ass’), Informal: forsee and avoid the possibility of attack or criticism. (Oxford Dictionary of Idoms)
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Ken G – June 18, 2003

Regarding the grass/lawnmower threat, our sergeants would tell us peons to straighten up or else "your ass will be grass and I'll be the lawnmower and we'll go 'round and 'round!" I always understood it to mean that Sarge would be heavy with the punishment if we didn't comply.

Tom Malone, Spring Hill, Florida

Not to be confused with "your ass is gross"

is your ass is grass is a huge offencesive word or something small? my sub- trainer said that to me

dave

it comes from stability challenged donkeys on steep hills. They topple on their ass. Hence the accepted meaning: in trouble

How about ass or grass, as in hitchhiking, give me sex or marijuanna. Are these connected ya think?

cover me man im going in, or your ass is grass.