🔼The name Jashar in the Bible
The name Jashar occurs only once in the Bible, in the phrase "the Book of Jashar". This extra-canonical and not extant book is mentioned twice in the Bible:
- In Joshua 10:13 the Book of Jashar is listed as an authoritative reference that also reported the miraculous behavior of the sun in the valley of Gibeon and the moon in the valley of Aijalon, during Joshua's battle against the Amorite king Adoni-zedek.
- In 2 Samuel 1:18, the Book of Jashar is mentioned to contain the Song of the Bow; David's gut wrenching lament over Jonathan.
There's no telling when exactly the Book of Jashar was lost but it seems it was a collection of very old Hebrew poems. Some scholars propose that also Solomon's dedication of the temple (1 Kings 8:12-13) and the Song of Deborah and Barak (Judges 5) were originally first bundled in the Book of Jashar and later incorporated into the Bible.
🔼Etymology of the name Jashar
The Encyclopedia Britannica deems the etymology of Jashar uncertain, but that's a bit harsh. Most scholars understand it to come from the verb ישר (yashar), meaning to be straight:
The name Jashar means Upright.
That is, if Jashar actually is a name. Most scholars don't think so and both the Septuagint and the Vulgate don't speak of the Book of Jashar but the Book of the Upright (biblion tou euthous in Greek, hence our words ethics and ethical, and liber justorum in Latin, hence our words just and justice).
Why pretty much every modern translation of the Bible insists on speaking of a Book of Jashar and not a Book of the Upright is a mystery.