Literally: pillow – “spear”* – wait – sunrise
Alternately: Always ready to give battle. Never dropping your guard or slacking off from necessary preparations. “Sleeping with a weapon as your pillow, awaiting the break of dawn.”
Notes: This phrase comes to us from the Book of Jin (Japanese 『晋書』 = Shinjo), a Tang-era history that looked back at the Jin Dynasty over two hundred years prior. (Note that this is a different work from the History of Jin, which covers a different dynasty that rose and fell a thousand years later!) Apparently an official named Liu Kun (劉琨, Japanese Ryuu Kon) learned that his friend had been promoted ahead of him, and wrote a letter complaining about this and extolling his own virtue as a warrior so utterly prepared for action that he bedded down on top of his weapons.
As is often the case with Chinese-origin phrases, this may be expanded into a full sentence, 戈を枕にして旦を待つ (Hoko wo makura ni shite ashita wo matsu). Contrast 枕を高くして寝る.
* As a rule, 戈 is translated through “localization” using a Western weapon name: lance; spear; pike, halberd. The actual original design is something closer to a long-hafted pick: the pointy bit is mounted and braced on the haft so that it projects out perpendicularly, although later designs seem to have added a spear-like vertical spike as well. Wiktionary has a pretty good chart showing how the character evolved from a pure pictogram depicting this shape into its current form.