My wife has been baking bread lately (and very good it is too), and this morning as I was gazing fondly at the latest loaf the phrase “staff of life” popped into my head and I wondered about it. The metaphor seemed clear—a staple food is something you lean on—but still somewhat odd, and I wondered if there were a backstory. I went, of course, to the OED, where I found the phrase as definition 4c (first cite: 1638 PENKETHMAN Artach. Ajb, “Bread is worth all, being the Staffe of life,” where Artach. = Artachthos; or a new booke declaring the assise or weight of bread); definition 4b, which gave rise to it, is:
In the Biblical phr. to break the staff of bread (literally from Heb. maṭṭēh ‘leχem, Vulg. baculum panis), to diminish or cut off the supply of food.
1382, 1388 WYCLIF Lev. xxvi. 26. 1560 BIBLE (Geneva) Lev. xxvi. 26. Ps. cv. 16. Ezek. iv. 16. [And so 1611]. c1586 C’TESS PEMBROKE Ps. CV. iv, Scarse had he spoken, When famine came, the staff of bread was broken. 1596 BARLOW Three Serm. i. 121 God in his lawe threatneth that he will breake the staffe of bread, that is, bread shall not nourish them that eate it.
So it’s a metaphor specific to the Hebrew Bible that managed to get solidly rooted in the English language; a quick look through my dictionaries suggests English is unique in that respect—staff of life is defined by phrases that translate to ‘most important foodstuff,’ ‘support of life,’ and the like. (The Hebrew word mateh, incidentally, now [also] means ‘military staff’: mateh ha-klali ‘General Staff.’)
I close with a quote from a letter by the 14-year-old Emily Dickinson (Thursday, September 26. 1845, to her friend Abiah Root):
I am going to learn to make bread tomorrow. So you may imagine me with my sleeves rolled up mixing Flour, Milk, Saleratus &c with a deal of grace. I advise you if you dont know how to make the staff of life to learn with dispatch.
Saleratus (sal aeratus ‘aerated salt’) was a nineteenth-century form of baking powder; the stress is on the penult (sal-uh-RATE-us).
I note that the Italian translation appended to the letter at the linked site renders the final sentence, with “the staff of life,” as follows: “Se non sai come fare l’alimento primario ti raccomando di imparare in fretta.”