In the poem "Mending Wall," the term "old-stone savaged armed" is used to describe the neighbor. The speaker refers to the neighbor as an "old-stone savage armed" because he is old fashioned. He stands as a primitive man with stones in hand as if he is armed for battle. The neighbor has the notion that "good fences make good neighbors." The neighbor learned what he knows from his father.
The neighbor "moves in darkness" because he is so old fashioned. He believes in fences just as his father did. The neighbor will not let go of the past. He meets with the speaker each spring to mend the wall that has been broken down.
As the neighbor "moves in darkness" like an "old-stone savaged armed," he reminds the speaker of someone from ancient days--an old-stone savage armed." The neighbor is from another time period. He is so old fashioned until he sticks to his belief that "good fences make good neighbors." While the speaker does not see the need for a wall between them, the speaker humors the "old-stone savage armed" by helping to repair the wall.
Truly, the speaker sees the neighbor as someone from a long ago time period:
The speaker sees his uncommunicative neighbor as “an old-stone savage” who “moves in darkness” and seems incapable of thinking beyond the clichéd maxim, which the neighbor repeats, “Good fences make good neighbors.”