Bottom strength radical — the indexing radical. Pictograph of a muscular arm or a plow with bent handle, symbolizing applied power. Pairs with the slave-image above to give 'putting one's strength behind something' — striving, exerting. Family: 加 add, 助 help, 劲 vigor, 勇 brave.
Top 奴 (woman + hand) supplies the sound: nú → nǔ with tone shift. Same phonetic appears in 怒 anger, 弩 crossbow. Faintly contributes the idea of forced labor — a slave straining at a task — which dovetails with 努's sense of effortful exertion.
