While the origin isn't definitively known, it's likely rooted in medieval English usage. The word "sally" itself has a history of being associated with bold or sudden movements.
Here's what's known about its potential origins:
* Military context: "Sally" was used in a military context to describe a sudden sortie from a besieged fortress. This aligns with the idea of confidently venturing out into the unknown.
* Old French: "Saller" in Old French meant "to leap, spring, or jump." This reinforces the idea of quick and decisive action.
Therefore, "Sally forth" likely evolved from a combination of these historical usages, combining the notions of boldness, sudden movement, and a sense of venturing into the unknown.
It's worth noting that "sally forth" isn't an extremely common phrase today. However, its use in literature and its clear meaning make it a recognizable and sometimes playful expression.