Joseph is the bow-tribe, according to Genesis 49:23-24: Though attached by archers, his hands are strong to respond.
Psalm 78:9 tells us that Ephraim, the largest sub-tribe of Joseph, is also a tribe of bowmen. But, referring to a battle recorded in Judges 20, the Psalm indicates that Ephraim’s bowmen are unreliable. They “turned back” (haphak) in the day of battle, and thus became a leading example of the stubborn and rebellious fathers (Psalm 78:8).
The Psalm refers again to bows in verse 57, and again uses the verb haphak to describe the unfaithfulness of the fathers: they “turned back and dealt unfaithfully like their fathers; they were turned aside like a deceitful (remiyyah, also ‘indolent’) bow.” In this case, though, the turning back is not a refusal to engage in battle but rather idolatry: “they provoked Him to anger with their high places, and moved Him to jealousy with their graven images” (v. 58).
The two references to the “bow” in the Psalm are linked. Ephraim’s bows are indolent because they don’t follow through in their commitment to fight. Despite being equipped with bows, they don’t offer any help in battle. That is not simply cowardice but idolatry, a failure to trust in Yahweh. Idolatry will produce indolent bowmen.