Left indexing radical 石 (stone) - supplies the meaning. Stones are solid, hard, immovable: the perfect metaphor for what is certain and unwavering. 'Solid as a rock' becomes 'definite, sure.' The stone-radical also covers other firmness words: 硬 (hard), 碑 (stele), 砖 (brick).
Right component 角 (horn) supplies the sound (jiǎo → què, with drift; traditional 確 used 隺, a closer phonetic). The simplified 确 swapped to 角 to streamline writing. Image: hard as stone, sharp as a horn - a doubly-firm certainty.
