The root of haṣartu is h-ṣ-r (ه ص ر), associated with bending, pressing, or pulling something so that it yields. With the first-person suffix, it is a perfect verb, first-person singular: “I pulled/bent.”
In bi-fūday raʾsihā, fūd denotes the hair at a side of the head or around the temple. Fūday is dual: after the preposition bi- it appears in the genitive with yāʾ, and the dual nūn is dropped because it is the first term of a construct. Raʾsihā completes that construct.
The expression combines verb, point of contact, dual inflection, and iḍāfah. Taking fūday as singular would obscure the image of the two sides of the head.