How far along a dashed line its pattern begins. A named drawing property rather than a concept.
English compound of dash (Middle English daschen, to strike, dart) + offset (off + set, a shifted starting point); how far along a dashed line's own repeating pattern its drawing begins.
It borrows dashed's own strike and pairs it with a shift, naming only where in that repeating strike the drawing happens to begin. There is no idea behind the word beyond the number it holds.