The phrase (as) cold as, or colder than, Pharaoh’s heart means extremely cold.
—Synonym: (as) cold as a stepmother’s breath.
The earliest occurrences that I have found of an earlier phrase, (as) hard as, or harder than, Pharaoh’s heart, meaning extremely hard, are as follows:
1-: From Hard Times, published in the Woodstock Observer (Woodstock, Vermont) of Wednesday 30th September 1829:
The times are villanously [sic] hard. If any one desires proof of the assertion, let him thrust a hand into his empty pocket—or if it be not empty, filled with duns harder than Pharaoh’s heart, and bills longer than a snipe’s.
2-: From The United States Gazette (Philadelphia, Pennsylvania) of Saturday 17th January 1835:
The ice boat in Baltimore, has been employed every day since the cold weather commenced. It cut through ice a foot thick, and as hard as Pharaoh’s heart, with great ease, and at the same time dragged large vessels out into Chesapeake.
Both the phrases (as) hard as, or harder than, Pharaoh’s heart, and (as) cold as, or colder than, Pharaoh’s heart refer to the kings of ancient Egypt mentioned in the Old Testament and Hebrew Scriptures, in whose time the oppression and Exodus of the Israelites took place—specifically to the hardening of Pharaoh’s heart in the Book of Exodus, 7:13-22, which is as follows in the King James Bible (1611):
13 And hee hardened Pharaohs heart, that hee hearkened not vnto them, as the Lord had said.
14 And the Lord saide vnto Moses, Pharaohs heart is hardened: he refuseth to let the people goe.
15 Get thee vnto Pharaoh in the morning, loe, he goeth out vnto the water, and thou shalt stand by the riuers brinke, against hee come: and the rod which was turned to a serpent, shalt thou take in thine hand.
16 And thou shalt say vnto him, The Lord God of the Hebrewes hath sent me vnto thee, saying; Let my people goe, that they may serue mee in the wildernesse: and beholde, hitherto thou wouldest not heare.
17 Thus saith the Lord, In this thou shalt know that I am the Lord: behold, I will smite with the rod that is in my hand, vpon the waters which are in the riuer, and they shalbe turned to blood.
18 And the fish that is in the riuer shall die, and the riuer shall stincke, and the Egyptians shall loathe to drinke of the water of the riuer.
19 And the Lord spake vnto Moses, Say vnto Aaron, Take thy rod, & stretch out thine hand vpon the waters of Egypt, vpon their streames, vpon their riuers, and vpon their ponds, and vpon all their pooles of water, that they may become blood, and that there may be blood throughout all the land of Egypt, both in vessels of wood, and in vessels of stone.
20 And Moses and Aaron did so, as the Lord commanded: and he lift vp the rod and smote the waters that were in the riuer, in the sight of Pharaoh, and in the sight of his seruants: and all the waters that were in the riuer, were turned to blood.
21 And the fish that was in the riuer died: and the riuer stunke, and the Egyptians could not drinke of the water of the riuer: and there was blood throughout all the land Egypt.
22 And the Magicians of Egypt did so, with their enchantments: and Pharaohs heart was hardened, neither did he hearken vnto them, as the Lord had said.
These are, in chronological order, the earliest occurrences of the phrase as cold as, or colder than, Pharaoh’s heart that I have found:
1-: From American Girls in Italy, by ‘M. J. E.’, published in The New North-West (Deer Lodge, Montana) of Friday 7th April 1876:
I am sure Assisi must have been built immediately after the flood and the original houses of the natives are standing, and will stand until the last trump shall sound. They are built for strength and not for beauty, their founders made a success of their work, but the amount of brick and stone which go to each edifice would cause a modern builder’s soul to groan over the sinful waste of good material. I shall never forget the chill that mounted to my very marrow as we mounted the wide tiled staircase of the inn to our rooms, also tile-floored and colder than Pharaoh’s heart.
2-: From Serena and Samantha: Being a Chronicle of Events at the Torbolton Home (Boston (Massachusetts): Sherman, French & Company, 1912), by Rosa Kellen Hallett:
It was June. At least so it was termed in the almanac, but it was following straitly in the traditions of that preceding month of May which had been, to quote Mrs. Dodd, “as cold as Pharaoh’s heart.”
3-: From Hitch-hiking to Battlefront Has Its Points—Cold Ones, by Inez Robb, International News Service War Correspondent, published in The Courier-Journal (Louisville, Kentucky) of Wednesday 17th February 1943:
An Advanced Air Base In North Africa, Feb. 16.—In company with 2,600 pounds of frozen prime beef and a mixed group of American soldiers trying to thumb their way to the Tunisian front, I am flying today toward an advanced air base in North Africa in a plane christened the “Olive Goose.” […]
I am bundled up in all the clothing I own, including two sets of red flannel underwear, but nothing, not even red flannel, will ever warm this frigid aluminum bucket seat in which I am shivering.
Fighter escorts brought me all the way. As the plane sets down here at an advanced airbase in North Africa I ought to feel noble or heroic or something. But darn—I just feel cold, hungry and tired. And that aluminum bucket is still as cold as was Pharaoh’s heart.
4-: From Spider House (Kingston (New York): Handi-Book Editions, 1949), a mystery novel by the U.S. author Francis Van Wyck Mason (1901-1978):
Clump! Clump! Sergeant Lynch was moving around. Presently came the sound of his voice, peevish and sleepy. “Damn, it’s cold as Pharaoh’s heart what with the draught on these bloody stairs.”
5-: From A little off center, by Louise Fleming, published in The Baxter Bulletin (Mountain Home, Arkansas) of Thursday 30th December 1971:
Tuesday evening saw another skirmish in my continuing war with a mechanized world. Charley Howard left the office a short time before I did on Tuesday, and before starting for his home in Dora, Mo., he very kindly started the motor on my venerable Ford, which has a temperature range as cold as Pharaoh’s heart. Consequently, upon leaving work I entered a car whose motor was running smoothly and reassuringly.