刚

Right knife radical (Kangxi #18, vertical-knife form of 刀) - the indexing radical. The 'cutting/blade' sense is metaphorical here: a hard knife-edge is the model of unyielding firmness. Same radical anchors 刻, 划, 别, 剩 - the cutting-action family. 刚 = 'ridge-hard like a blade' - fits both 'just now (sharp moment)' and 'firm/unyielding'.

Left component supplies the sound (gāng, exact match) and a faint semantic echo: 冈 is the crest of a ridge - solid, hard rock - which fits the 'firm, unyielding' meaning of 刚. Same phonetic in 钢 (steel), 纲 (mainstay), 岗 (post). The simplified 冈 stands in for the original 岡; both are 4-stroke ridge silhouettes.