I’m interested in the line, “Raising tax rates on the wealthy is Obama’s line in the sand” in the following lead-copy of Washington Post’s (December 4) article titled “President Obama’s tough time taking a hard line on the fiscal cliff.”
“The president has done something that could make negotiations far more difficult. He has made it clear that raising tax rates on the wealthy is his line in the sand.”
I see the idiom, “Bury your head in the sand” in Cambridge online dictionary, and “Head in the sand” in Merriam-Webster online dictionary, but I cannot find “in the sand” as an idiom in both dictionaries. Oxford online dictionary registers only “the sands (of time) are running out” as a sand-related phrase.
On the other hand, Google Ngram shows consistent incidences of “in the sand” since cir 1840. I guess “his line in the sand” means “his statement (promise) unachievable like “a castle in the air.” Is “something (letter, record, plan, promise, structure) in the sand” a popular expression? If so, why isn’t it registered in major dictionaries as a simile of fragility, ephemerality, or easy-erasable like the title of Japanese popular song, "A love-letter writen on the sand"?